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Astronomical  Drawings 


MANUAL 


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THE 

TROUVELOT 

ASTRONOMICAL  DRAWINGS 

MANUAL 


THE 


TROUVELOT 


Astronomical  Drawings 

MANUAL 


BY 


E.   L.  TROUVELOT, 


FORMERLY  CONNECTED  WITH   THE   OBSERVATORY   OF   HARVARD   COLLEGE  ;    FELLOW   OF   THE 
AMERICAN    ACADEMY   OF   ARTS   AND   SCIENCES,    AND   MEMBER   OF  THE   SELENO- 
GRAPHICAL    SOCIETY    OF     GREAT    BRITAIN  ;    IN     CHARGE    OF     A 
GOVERNMENT      EXPEDITION     TO     OBSERVE      THE 
TOTAL    SOLAR    ECLIPSE   OF    1878. 


NEW    YO/EK 

CHARLES    SCRpNER'S    SONS 
18/2 


T7 

Astron. 


ASTRONOMY 


COPSTKIGHT  BY 

CHABLES  SCElBNEE'S  SONS 

1882 


NEW   YORK 

JENKINS     A     THOMAS,     PRINTERS 
8  8PBUCE   8TEEET 


INTRODUCTION. 


DURING  a  study  of  the  heavens,  which  has  now  been  continued  for 
more  than  fifteen  years,  I  have  made  a  large  number  of  observations 
pertaining  to  physical  astronomy,  together  with  many  original 
drawings  representing  the  most  interesting  celestial  objects  and 
phenomena. 

With  a  view  to  making  these  observations  more  generally  useful, 
I  was  led,  some  years  ago,  to  prepare,  from  this  collection  of  draw- 
ings, a  series  of  astronomical  pictures,  which  were  intended  to  repre- 
sent the  celestial  phenomena  as  they  appear  to  a  trained  eye  and  to 
an  experienced  draughtsman  through  the  great  modern  telescopes, 
provided  with  the  most  delicate  instrumental  appliances.  Over  two 
years  were  spent  in  the  preparation  of  this  series,  which  consisted  of 
a  number  of  large  drawings  executed  in  pastel.  In  1876,  these  draw- 
ings were  displayed  at  the  United  States  Centennial  Exhibition  at 
Philadelphia,  forming  a  part  of  the  Massachusetts  exhibit,  in  the 
Department  of  Education  and  Science. 

The  drawings  forming  the  present  series  comprise  only  a  part  of 
those  exhibited  at  Philadelphia  ;  but,  although  fewer  in  number, 
they  are  quite  sufficient  to  illustrate  the  principal  classes  of  celestial 
objects  and  phenomena. 

While  my  aim  in  this  work  has  been  to  combine  scrupulous  fidel- 
ity and  accuracy  in  the  details,  I  have  also  endeavored  to  preserve 
the  natural  elegance  and  the  delicate  outlines  peculiar  to  the  objects 
depicted  ;  but  in  this,  only  a  little  more  than  a  suggestion  is  possi- 
ble, since  no  human  skill  can  reproduce  upon  paper  the  majestic 
beauty  and  radiance  of  the  celestial  objects. 

The  plates  were  prepared  under  my  supervision,  from  the  origi- 
nal pastel  drawings,  and  great  care  has  been  taken  to  make  the 
reproduction  exact. 

The  instruments  employed  in  the  observations,  and  in  the  deline- 
ation of  the  heavenly  bodies  represented  in  the  series,  have  varied 


670903 


VI 


in  aperture  from  6  to  26  inches,  according  to  circumstances,  and  to 
the  nature  of  the  object  to  be  studied.  The  great  Washington  re- 
fractor, kindly  placed  at  my  disposal  by  the  late  Admiral  C.  H. 
Davis,  has  contributed  to  this  work,  as  has  also  the  26  inch  telescope 
of  the  University  of  Virginia,  while  in  the  hands  of  its  celebrated 
constructors,  Alvan  Clark  &  Sons.  The  spectroscope  used  was 
made  by  Alvan  Clark  &  Sons.  Attached  to  it  is  an  excellent  dif- 
fraction grating,  by  Mr.  L.  M.  Rutherfurd,  to  whose  kindness  I  am 
indebted  for  it. 

Those  unacquainted  with  the  use  of  optical  instruments  generally 
suppose  that  all  astronomical  drawings  are  obtained  by  the  photo- 
graphic process,  and  are,  therefore,  comparatively  easy  to  procure  ; 
but  this  is  not  true.  Although  photography  renders  valuable  assist- 
ance to  the  astronomer  in  the  case  of  the  Sun  and  Moon,  as  proved 
by  the  fine  photographs  of  these  objects  taken  by  M.  Janssen  and 
Mr.  Rutherfurd  ;  yet,  for  other  subjects,  its  products  are  in  general 
so  blurred  and  indistinct  that  no  details  of  any  great  value  can  be 
secured.  A  well-trained  eye  alone  is  capable  of  seizing  the  deli- 
cate details  of  structure  and  of  configuration  of  the  heavenly  bodies, 
which  are  liable  to  be  affected,  and  even  rendered  invisible,  by  the 
slightest  changes  in  our  atmosphere. 

The  method  employed  to  secure  correctness  in  the  proportions 
of  the  original  drawings  is  simple,  but  well  adapted  to  the  purpose 
in  view.  It  consists  in  placing  a  fine  reticule,  cut  on  glass,  at  the 
common  focus  of  the  objective  and  the  eye-piece,  so  that  in  viewing 
an  object,  its  telescopic  image,  appearing  projected  on  the  reticule, 
can  be  drawn  very  accurately  on  a  sheet  of  paper  ruled  with  corre- 
sponding squares.  For  a  series  of  such  reticules  I  am  indebted  to 
the  kindness  of  Professor  William  A.  Rogers,  of  the  Harvard  College 
Observatory. 

The  drawings  representing  telescopic  views  are  inverted,  as  they 
appear  in  a  refracting  telescope — the  South  being  upward,  the  North 
downward,  the  East  on  the  right,  and  the  West  on  the  left.  The 
Comet,  the  Milky- Way,  the  Eclipse  of  the  Moon,  the  Aurora  Borea- 
lis,  the  Zodiacal  Light  and  the  Meteors  are  represented  as  seen 
directly  in  the  sky  with  the  naked  eye.  The  Comet  was,  however, 
drawn  with  the  aid  of  the  telescope,  without  which  the  delicate 
structure  shown  in  the  drawing  would  not  have  been  visible. 

The  plate  representing  the  November  Meteors,  or  so-called 
"  Leonids,"  may  be  called  an  ideal  view,  since  the  shooting  stars 
delineated,  were  not  observed  at  the  same  moment  of  time,  but  dur- 
ing the  same  night.  Over  three  thousand  Meteors  were  observed 


Vll 

between  midnight  and  five  o'clock  in  the  morning  of  the  day  on 
which  this  shower  occurred  ;  a  dozen  being  sometimes  in  sight  at 
the  same  instant.  The  paths  of  the  Meteors,  whether  curved,  wavy, 
or  crooked,  and  also  their  delicate  colors,  are  in  all  cases  depiqted 
as  they  were  actually  observed. 

In  the  Manual,  I  have  endeavored  to  present  a  general  outline 
of  what  is  known,  or  supposed,  on  the  different  subjects  and  phe- 
nomena illustrated  in  the  series.  The  statements  made  are  derived 
either  from  the  best  authorities  on  physical  astronomy,  or  from  my 
original  observations,  which  are,  for  the  most  part,  yet  unpublished. 

The  figures  in  the  Manual  relating  to  distance,  size,  volume, 
mass,  etc.,  are  not  intended  to  be  strictly  exact,  being  only  round 
numbers,  which  can,  therefore,  be  more  easily  remembered. 

It  gives  me  pleasure  to  acknowledge  that  the  experience  ac- 
quired in  making  the  astronomical  drawings  published  in  Volume 
VIII.  of  the  Annals  of  the  Harvard  College  Observatory,  while 
I  was  connected  with  that  institution,  has  been  of  considerable  as- 
sistance to  me  in  preparing  this  work  ;  although  no  drawings  made 
while  I  was  so  connected  have  been  used  for  this  series. 

E.  L.  TROUVELOT. 

Cambridge,  March,  1882. 


CONTENTS. 


PAGE 

INTKODUCTION v 

LIST  OF  PLATES xi 

THE  SUN. 

GENERAL  REMARKS  ON  THE  SUN 1 

SUN-SPOTS  AND  VEILED  SPOTS 8 

SOLAR  PROTUBERANCES 18 

TOTAL  ECLIPSE  OF  THE  SUN 23 

THE  AURORAL  AND  ZODIACAL  LIGHTS. 

THE  AURORA  BOREALIS 28 

THE  ZODIACAL  LIGHT 36 

THE  MOON. 

THE  MOON 42 

ECLIPSES  OF  THE  MOON 53 

THE  PLANETS. 

THE  PLANETS 57 

THE  PLANET  MARS 61 

THE  PL  YNET  JUPITER 73 

THE  PLANET  SATURN.  .                                                                                           .  83 


X 

COMETS  AND  METEORS. 

COMETS 99 

SHOOTING-STABS  AND  METEORS 115 

THE  STELLAR  SYSTEMS. 

THE  MILKY- WAY  OR  GALAXY 128 

THE  STAR-CLUSTERS 137 

THE  NEBULJS 144 

APPENDIX ,       . .  159 


LIST    OF    PLATES. 


Plate     I.         GROUP  OF  SUN-SPOTS  AND  VEILED  SPOTS. 

Observed  June  17,  1875,  atjh.  jom.  A.  M. 

"       II.        SOLAR  PROTUBERANCES. 

Observed  May  5,1873,  at  yh.  4om.  A.  M. 

"       III.      TOTAL  ECLIPSE  OF  THE  SUN. 

Observed  July  29,  i8j8,  at  Creston,   Wyoming  Territory. 
"       IV.      AURORA  BOREALIS. 

As  observed  March  I,  1872,  at  yh.  2$m.  P.  M. 
"       V.        THE  ZODIACAL  LIGHT. 

Observed  February  20,  1876. 

VI.  MARE  HUMORUM. 

From  a  study  made  in  187 5. 

VII.  PARTIAL  ECLIPSE  OF  THE  MOON. 

Observed  October  24,  1874. 

VIII.  THE  PLANET  MARS. 

Observed  September  3,  1877,  at  I ih.  55™.  P.  M. 
*'       IX.       THE  PLANET  JUPITER. 

Observed  November  i,  1880,  at  yh.  jom.  P.  M. 
*'       X.         THE  PLANET  SATURN. 

Observed  November  jo,  1874,  at  $h.  $om.  P.  M. 
XI.       THE  GREAT  COMET  OF  1881. 

Observed  on  the  night  of  June  25-26,  at  ih.  jom.  A.  M. 

*  For  Key  to  the  Plates,  see  Appendix. 


Xll 

Plate  XII.     THE  NOVEMBER  METEORS. 

As  observed  between  midnight  and  5  o'clock  A.  M.,  on  the  night 
of  November  13-14,  1868. 

XIII.    PART  OF  THE  MILKV-WAY. 

From  a  study  made  during  the  years  1874,  1875  and  1876. 
"       XIV.     STAR-CLUSTER  IN  HERCULES. 

From  a  study  made  in  June,  1877. 
"       XV.      THE  GREAT  NEBULA  IN  ORION. 

From  a  study  made  in  the  years  1875-76. 


%*  Reproduced  from  the  Original  Drawings,  by  Armstrong    6f    Company, 
Riverside  Press,  Cambridge,  Mass. 


GENERAL  REMARKS  ON  THE  SUK 


THE  Sun,  the  centre  of  the  system  which  bears  its  name,  is  a  self- 
luminous  sphere,  constantly  radiating  heat  and  light. 

Its  apparent  diameter,  as  seen  at  its  mean  from  the  Earth,  sub- 
tends an  angle  of  32',  or  a  little  over  half  a  degree.  A  dime,  placed 
about  six  feet  from  the  eye,  would  appear  of  the  same  proportions, 
and  cover  the  Sun's  disk,  if  projected  upon  it. 

That  the  diameter  of  the  Sun  does  not  appear  larger,  is  due  to 
the  great  distance  which  separates  us  from  that  body.  Its  distance 
from  the  Earth  is  no  less  than  92,000,000  miles.  To  bridge  this 
immense  gap,  would  require  11,623  globes  like  the  Earth,  placed 
side  by  side,  like  beads  on  a  string. 

The  Sun  is  an  enormous  sphere  whose  diameter  is  over  108  times 
the  diameter  of  our  globe,  or  very  nearly  860,000  miles.  Its  radius  is 
nearly  double  the  distance  from  the  Earth  to  the  Moon.  If  we  sup- 
pose, for  a  moment,  the  Sun  to  be  hollow,  and  our  globe  to  be  placed 
at  the  centre  of  this  immense  spherical  shell,  not  only  could  our 
satellite  revolve  around  us  at  its  mean  distance  of  238,800  miles,  as 
now,  but  another  satellite,  placed  190,000  miles  farther  than  the 
Moon,  could  freely  revolve  likewise,  without  ever  coming  in  con- 
tact with  the  solar  envelope. 

The  circumference  of  this  immense  sphere  measures  2,800,000 
miles.  While  a  steamer,  going  at  the  rate  of  300  miles  a  day,  would 
circumnavigate  the  Earth  in  83  days,  it  would  take,  at  the  same  rate, 
nearly  25  years  to  travel  around  the  Sun. 

The  surface  of  the  Sun  is  nearly  12,000  times  the  surface  of  the 
Earth,  and  its  volume  is  equal  to  1,300,000  globes  like  our  own.  If 
all  the  known  planets  and  satellites  were  united  in  a  single  mass,  600 
such  compound  masses  would  be  needed  to  equal  the  volume  of  our 
luminary. 

Although  the  density  of  the  Sun  is  only  one-quarter  that  of  the 
Earth,  yet  the  bulk  of  this  body  is  so  enormous  that,  to  counterpoise 
it,  no  less  than  314,760  globes  like  our  Earth  would  be  required. 


2  '  THE    TROUVELOT 

The  Sun  uniformly  revolves  around  its  axis  in  about  2$%  days. 
Its  equator  is  inclined  7°  15'  to  the  plane  of  the  ecliptic,  the  axis  of 
rotation  forming,  therefore,  an  angle  of  82°  45'  with  the  same  plane. 
As  the  Earth  revolves  about  the  Sun  in  the  same  direction  as  that 
of  the  Sun's  rotation,  the  apparent  time  of  this  rotation,  as  seen  by 
a  terrestrial  spectator,  is  prolonged  from  25^  days  to  about  27  days 
and  7  hours. 

The  rotation  of  the  Sun  on  its  axis,  like  that  of  the  Earth  and 
the  other  planets,  is  direct,  or  accomplished  from  West  to  East.  To 
an  observer  on  the  Earth,  looking  directly  at  the  Sun,  the  rotation 
of  this  body  is  from  left  to  right,  or  from  East  to  West. 

The  general  appearance  of  the  Sun  is  that  of  an  intensely  lumin- 
ous disk,  whose  limb,  or  border,  is  sharply  defined  on  the  heavens. 
When  its  telescopic  image  is  projected  on  a  screen,  or  fixed  on 
paper  by  photography,  it  is  noticed  that  its  disk  is  not  uniformly 
bright  throughout,  but  is  notably  more  luminous  in  its  central  parts. 
This  phenomenon  is  not  accidental,  but  permanent,  and  is  due  in 
reality  to  a  very  rare  but  extensive  atmosphere  which  surrounds 
the  Sun,  and  absorbs  the  light  which  that  body  radiates,  propor- 
tionally to  its  thickness,  which,  of  course,  increases  towards  the 
limb,  to  an  observer  on  the  Earth. 

THE  ENVELOPING  LAYERS  OF  THE  SUN. 

The  luminous  surface  of  the  Sun,  or  that  part  visible  at  all  times, 
and  which  forms  its  disk,  is  called  the  Photosphere,  from  the  property 
it  is  supposed  to  possess  of  generating  light.  The  photosphere  does 
not  extend  to  a  great  depth  below  the  luminous  surface,  but  forms 
a  comparatively  thin  shell,  3,000  or  4,000  miles  thick,  which  is  dis- 
tinct from  the  interior  parts,  above  which  it  seems  to  be  kept  in  sus- 
pense by  internal  forces.  From  the  observations  of  some  astrono- 
mers it  would  appear  that  the  diameter  of  the  photosphere  is  subject 
to  slight  variations,  and,  therefore,  that  the  solar  diameter  is  not  a 
constant  quantity.  From  the  nature  of  this  envelope,  such  a  result 
does  not  seem  at  all  impossible,  but  rather  probable. 

Immediately  above  the  photosphere  lies  a  comparatively  thin 
stratum,  less  than  a  thousand  miles  in  thickness,  called  the  Revers- 
ing Layer.  This  stratum  is  composed  of  metallic  vapors,  which,  by 
absorbing  the  light  of  particular  refrangibilities  emanating  from  the 
photosphere  below,  produces  the  dark  Fraunhofer  lines  of  the  solar 
spectrum. 

Above  the  reversing  layer,  and  resting  immediately  upon  it,  is  a 


ASTRONOMICAL    DRAWINGS.  3 

shallow,  semi-transparent  gaseous  layer,  which  has  been  called  the 
Chromosphere^  from  the  fine  tints  which  it  exhibits  during  total 
eclipses  of  the  Sun,  in  contrast  with  the  colorless  white  light  radiated 
by  the  photosphere  below.  Although  visible  to  a  certain  extent  on 
the  disk,  the  chromosphere  is  totally  invisible  on  the  limb,  except 
with  the  spectroscope,  and  during  eclipses,  on  account  of  the  nature 
of  its  light,  which  is  mainly  monochromatic,  and  too  feeble,  com- 
pared with  that  emitted  by  the  photosphere,  to  be  seen. 

The  chromospheric  layer,  which  has  a  thickness  of  from  3,000  to 
4,000  miles,  is  uneven,  and  is  usually  upheaved  in  certain  regions, 
its  matter  being  transported  to  considerable  elevations  above  its 
general  surface,  apparently  by  some  internal  forces.  The  portions 
of  the  chromosphere  thus  lifted  up,  form  curious  and  complicated 
figures,  which  are  known  under  the  names  of  Solar  Protuberances,  or 
Solar  Flames. 

Above  the  chromosphere,  and  rising  to  an  immense  but  unknown 
height,  is  the  solar  atmosphere  proper,  which  is  only  visible  during 
total  eclipses  of  the  Sun,  and  which  then  surrounds  the  dark  body 
of  the  Moon  with  the  beautiful  rays  and  glorious  nimbus,  called  the 
Corona. 

These  four  envelopes  :  the  photosphere,  the  reversing  layer,  the 
chromosphere,  and  the  corona,  constitute  the  outer  portions  of  our 
luminary. 

Below  the  photosphere  little  can  be  seen,  although  it  is  known, 
as  will  appear  below,  that  at  certain  depths  cloud-like  forms  exist, 
and  freely  float  in  an  interior  atmosphere  of  invisible  gases.  Beyond 
this  all  is  mystery,  and  belongs  to  the  domain  of  hypothesis. 

STRUCTURE    OF   THE   PHOTOSPHERE   AND   CHROMOSPHERE. 

The  apparent  uniformity  of  the  solar  surface  disappears  when  it 
is  examined  with  a  telescope  of  sufficient  aperture  and  magnifying 
powers.  Seen  under  good  atmospheric  conditions,  the  greater  part 
of  the  solar  surface  appears  mottled  with  an  infinite  number  of 
small,  bright  granules,  irregularly  distributed,  and  separated  from 
each  other  by  a  gray-tinted  background. 

These  objects  are  known  under  different  names.  The  terms 
granules  and  granulations  answer  very  well  for  the  purpose,  as  they 
do  not  imply  anything  positive  as  to  their  form  and  true  nature. 
They  have  also  been  called  Luculce,  Rice  Grains,  Willow  Leaves,  etc., 
by  different  observers. 

Although  having  different  shapes,  the  granulations  partake  more 


4  THE     TROUVELOT 

or  less  of  the  circular  or  slightly  elongated  form.  Their  diameter, 
which  varies  considerably,  has  been  estimated  at  from  o".5  to  3",  or 
from  224  to  1, 344  miles.  The  granulations  which  attain  the  largest 
size  appear,  under  good  atmospheric  conditions,  to  be  composed  of 
several  granules,  closely  united  and  forming  an  irregular  mass,  from 
which  short  appendages  protrude  in  various  directions. 

The  number  of  granulations  on  the  surface  of  the  Sun  varies  con- 
siderably under  the  action  of  unknown  causes.  Sometimes  they  are 
small  and  very  numerous,  while  at  other  times  they  are  larger,  less 
numerous,  and  more  widely  separated.  Other  things  being  equal, 
the  granulations  are  better  seen  in  the  central  regions  of  the  Sun 
than  they  are  near  the  limb. 

Usually  the  granulations  are  very  unstable  ;  their  relative  posi- 
tion, form,  and  size  undergoing  continual  changes.  Sometimes  they 
are  seen  to  congregate  or  to  disperse  in  an  instant,  as  if  acting  under 
the  influence  of  attractive  and  repulsive  forces  ;  assembling  in 
groups  or  files,  and  oftentimes  forming  capricious  figures  which  are 
very  remarkable,  but  usually  of  short  duration.  In  an  area  of  great 
solar  disturbances,  the  granulations  are  often  stretched  to  great  dis- 
tances, and  form  into  parallel  lines,  either  straight,  wavy,  or  curved, 
and  they  have  then  some  resemblance  to  the  flowing  of  viscous 
liquids. 

The  granulations  are  usually  terminated  either  by  rounded  or 
sharply  pointed  summits,  but  they  do  not  all  rise  to  the  same  height, 
as  can  be  ascertained  with  the  spectroscope  when  they  are  seen 
sidewise  on  the  limb.  In  the  regions  where  they  are  most  abundant, 
they  usually  attain  greater  elevations,  and  when  observed  on  the 
limb  with  the  spectroscope,  they  appear  as  slender  acute  flames. 

The  granulations  terminated  by  sharply-pointed  crests,  although 
observed  in  all  latitudes,  seem  to  be  characteristic  of  certain  regions. 
A  daily  study  of  the  chromosphere,  extending  over  a  period  of  ten 
years,  has  shown  me  that  the  polar  regions  are  rarely  ever  free  from 
these  objects,  which  are  less  frequent  in  other  parts  of  the  Sun.  In 
the  polar  regions  they  are  sometimes  so  abundant  that  they  com- 
pletely form  the  solar  limb.  These  forms  of  granulation  are  com- 
paratively rare  in  the  equatorial  zones,  and  when  seen  there,  they 
never  have  the  permanency  which  they  exhibit  in  the  polar  regions. 
When  observed  in  the  equatorial  regions,  they  usually  appear  in 
small  groups,  in  the  vicinity  of  sun  spots,  or  they  are  at  least  en- 
closed in  areas  of  disturbances  where  such  spots  are  in  process  of 
formation.  In  these  regions  they  often  attain  greater  elevations 
than  those  seen  in  high  latitudes. 


ASTRONOMICAL    DRA  WINGS.  5 

As  we  are  certain  that  in  the  equatorial  zones  these  slender 
flames  (Y.  *••.,  granulations)  are  a  sure  sign  of  local  disturbance,  it  may 
be  reasonably  supposed  that  the  same  kind  of  energy  producing 
them  nearly  always  prevails  in  the  polar  regions,  although  it  is 
there  much  weaker,  and  never  reaches  beyond  certain  narrow  limits. 

Studied  with  the  spectroscope,  the  granulations  are  found  to  be 
composed  in  the  main  of  incandescent  hydrogen  gas,  and  of  an  un- 
known substance  provisionally  called  "  helium."  Among  the  most 
brilliant  of  them  are  found  traces  of  incandescent  metallic  vapors, 
belonging  to  various  substances  found  on  our  globe. 

The  chromosphere  is  not  fixed,  but  varies  considerably  in  thick- 
ness in  its  different  parts,  from  day  to  day.  Its  thickness  is  usually 
greater  in  the  polar  regions,  where  it  sometimes  exceeds  6, 700  miles. 
In  the  equatorial  regions  the  chromosphere  very  rarely  attains  this 
height,  and  when  it  does,  the  rising  is  local  and  occupies  only  a 
small  area.  In  these  regions  it  is  sometimes  so  shallow  that  its 
depth  is  only  a  few  seconds,  and  is  then  quite  difficult  to  measure. 
These  numbers  give,  of  course,  the  extreme  limits  of  the  variations 
of  the  chromosphere  ;  but,  nearly  always,  it  is  more  shallow  in  the 
equatorial  regions  ;  and,  as  far  as  my  observations  go,  the  difference 
in  thickness  between  the  polar  and  equatorial  zones  is  greater  in 
years  of  calm  than  it  is  in  years  of  great  solar  activity.  But  ten 
years  of  observation  are  not  sufficient  to  warrant  any  definite  con- 
clusions on  this  subject. 

There  is  undoubtedly  some  relation  between  the  greater  thick- 
ness of  the  chromosphere  in  the  polar  regions,  and  the  abundance 
and  permanence  of  the  sharply-pointed  granulations  observed  in  the 
same  regions.  This  becomes  more  evident  when  we  know  that  the 
appearance  of  similarly-pointed  flames  in  the  equatorial  zones  is 
always  accompanied  with  a  local  thickening  of  the  chromosphere. 
The  thickening  in  the  polar  regions  may  be  only  apparent,  and  not 
due  to  a  greater  accumulation  of  chromospheric  gases  there  ;  but 
may  be  caused  by  some  kind  of  repulsive  action  or  polarity,  which 
lifts  up  and  extends  the  summit  of  the  granulations  in  a  manner 
similar  to  the  well-known  mode  of  electric  repulsion  and  polarity. 

As  it  seems  very  probable  that  the  heat  and  light  emanating 
from  the  Sun  are  mainly  generated  at  the  base  of  the  granulations,  in 
the  filamentary  elements  composing  the  chromosphere  and  photo- 
sphere, it  would  follow  that,  as  the  size  and  number  of  these  objects 
constantly  vary,  the  amount  of  heat  and  light  emitted  by  the  Sun 
should  also  vary  in  the  same  proportion. 

The  granulations  of  the  solar  surface  are  represented  on  Plate 


6  THE    TROUVELOT 

I.,  and  form  the  general  background  to  the  group  of  Sun-spots  form- 
ing the  picture. 

THE   FACUL^:. 

Although  the  solar  surface  is  mainly  covered  with  the  luminous 
granulations  and  the  grayish  background  above  described,  it  is  very 
rare  that  its  appearance  is  so  simple  and  uniform  as  already  repre- 
sented. For  the  most  part,  on  the  contrary,  it  is  diversified  by 
larger,  brighter,  and  more  complicated  forms,  which  are  especially 
visible  towards  the  border  of  the  Sun.  Owing  to  their  extraordinary 
brilliancy,  these  objects  have  been  called  Faculcz,  (torches.) 

Although  the  faculae  are  very  seldom  seen  well  beyond  50  helio- 
centric degrees  from  the  limb,  yet  they  exist,  and  are  as  numerous 
in  the  central  parts  of  the  disk  as  they  are  towards  the  border  ; 
since  they  form  a  part  of  the  solar  surface,  and  participate  in  its 
movement  of  rotation.  Their  appearance  near  the  limb  has  been  at- 
tributed to  the  effect  of  absorption  produced  by  the  solar  atmosphere 
on  the  light  from  the  photosphere  ;  but  this  explanation  seems  inad- 
equate, and  does  not  solve  the  problem.  The  well-known  fact  that 
the  solar  protuberances — which  are  in  a  great  measure  identical  with 
the  faculae — are  much  brighter  at  the  base  than  they  are  at  the 
summit,  perhaps  gives  a  clue  to  the  explanation  of  the  phe- 
nomenon; especially  since  we  know  that,  in  general,  the  summit  of 
the  protuberances  is  considerably  broader  than  their  base.  When 
these  objects  are  observed  in  the  vicinity  of  the  limb,  they  present 
their  brightest  parts  to  the  observer,  since,  in  this  position,  they  are 
seen  more  or  less  sidewise  ;  and,  therefore,  they  appear  bright  and 
distinct.  But  as  the  faculae  recede  from  the  limb,  their  sides,  being 
seen  under  a  constantly  decreasing  angle,  appear  more  and  more 
foreshortened  ;  and,  therefore,  these  objects  grow  less  bright  and 
less  distinct,  until  they  finally  become  invisible,  when  their  bases 
are  covered  over  by  the  broad,  dusky  summit  generally  terminating 
the  protuberances. 

The  faculae  appear  as  bright  and  luminous  masses  or  streaks  on 
the  granular  surface  of  the  Sun,  but  they  differ  considerably  in  form 
and  size.  Two  types  at  least  are  distinguishable.  In  their  simplest 
form  they  appear  either  as  isolated  white  spots,  or  as  groups  of  such 
spots  covering  large  surfaces,  and  somewhat  resembling  large  flakes 
of  snow.  In  their  most  characteristic  types  they  appear  as  intensely 
luminous,  heavy  masses,  from  which,  in  most  cases,  issue  intricate 
ramifications,  sometimes  extending  to  great  distances.  Generally, 
the  ramifications  issuing  from  the  masses  of  faculae  have  their  largest 


ASTRONOMICAL    DRA  WINGS.  1 

branches  directed  in  the  main  towards  the  eastern  limb  of  the  Sun. 
Some  of  these  branches  have  gigantic  proportions.  Occasionally 
they  extend  over  60°  and  even  80°  of  the  solar  surface,  and,  there- 
fore, attain  a  length  of  from  450,000  to  600,000  miles. 

Although  the  faculae  may  be  said  to  be  seen  everywhere  on  the 
surface  of  the  Sun,  there  is  a  vast  difference  in  different  regions, 
with  regard  to  their  size,  number,  and  brilliancy.  They  are  largest, 
most  abundant,  and  brightest  on  two  intermediate  zones  parallel  to 
the  solar  equator,  and  extending  35°  or  40°  to  the  north  and  to  the 
south  of  this  line.  The  breadth  of  these  zones  varies  considerably 
with  the  activity  of  the  solar  forces.  When  they  are  most  active, 
the  faculae  spread  on  either  side,  but  especially  towards  the  equator, 
where  they  sometimes  nearly  meet  those  of  the  other  zone.  In 
years  of  little  solar  activity  the  belts  formed  by  the  faculae  are  very 
narrow — the  elements  composing  them  being  very  few  and  small, 
although  they  never  entirely  disappear. 

The  faculae  are  very  unstable,  and  are  constantly  changing  : 
those  of  the  small  types  sometimes  form  and  vanish  in  a  few  min- 
utes. When  an  area  of  disturbance  of  the  solar  surface  is  observed 
for  some  time,  all  seems  in  confusion  ;  the  movements  of  the  granu- 
lations become  unusually  violent  ;  they  congregate  in  all  sorts  of 
ways,  and  thus  frequently  form  temporary  faculae.  Action  of  this 
kind  is,  for  the  most  part,  peculiar  to  the  polar  regions  of  the  Sun. 

The  larger  faculae  have  undoubtedly  another  origin,  as  they  seem 
to  be  mainly  formed  by  the  ejection  of  incandescent  gases  and 
metallic  vapors  from  the  interior  of  the  photosphere.  In  their  pro- 
cess of  development  some  of  the  heavy  masses  of  faculae  are  swollen 
up  to  great  heights,  being  torn  in  all  sorts  of  ways,  showing  large 
rents  and  fissures  through  which  the  sight  can  penetrate. 

Very  few  faculae  are  represented  in  Plate  I.  ;  several  streaks  are 
shown  at  the  upper  left-hand  corner,  some  appearing  as  whitish 
ramifications  among  the  granulations  representing  the  general  solar 
surface. 


THE    TROVVELOT 


SUN-SPOTS  AND  VEILED  SPOTS. 
PLATE  I. 

BESIDES  the  brilliant  faculae  already  described,  much  more  con- 
spicuous markings,  though  of  a  totally  different  character,  are  very 
frequently  observed  on  the  Sun.  On  account  of  their  darkish  ap- 
pearance, which  is  in  strong  contrast  with  the  white  envelope  of 
our  luminary,  these  markings  were  called  Macula,  or  Sun-spots,  by 
their  earlier  observers. 

The  Sun-spots  are  not  equally  distributed  on  the  solar  surface  ; 
but  like  the  faculae,  to  which  they  are  closely  related,  they  occupy  two 
zones — one  on  each  side  of  the  equator.  These  zones  are  comprised 
between  10°  and  35°  of  north  latitude,  and  10°  and  35°  of  south  lat- 
itude. Between  these  two  zones  is  a  belt  20°  in  width,  where  the 
Sun-spots  are  rarely  seen. 

Above  the  latitudes  35°  north  and  south,  the  Sun-spots  are  rare, 
and  it  is  only  occasionally,  and  during  years  of  great  solar  activity, 
that  they  appear  in  these  regions  ;  in  only  a  few  cases  have  spots 
of  considerable  size  been  seen  there.  A  few  observers,  however, 
have  seen  spots  as  far  as  40°  and  50°  from  the  equator ;  and  La 
Hire  even  observed  one  in  70°  of  north  latitude  ;  but  these  cases 
are  exceedingly  rare.  It  is  not  uncommon,  however,  to  see  very 
small  spots,  or  groups  of  such  spots,  within  8°  or  10°  from  the  poles. 

The  activity  of  the  Sun  is  subject  to  considerable  fluctuation,  and 
accordingly  the  Sun-spots  vary  in  size  and  number  in  different  years. 
During  some  years  they  are  large,  complicated,  and  very  numer- 
ous ;  while  in  others  they  are  'small  and  scarce,  and  are  sometimes 
totally  absent  for  weeks  and  months  together.  The  fluctuations  in 
the  frequency  of  Sun-spots  are  supposed  to  be  periodical  in  their 
character,  although  their  periods  do  not  always  appear  to  recur  at 
exactly  regular  intervals.  Sometimes  the  period  is  found  to  be  only 
nine  years,  while  at  other  times  it  extends  to  twelve  years.  The 
period  generally  adopted  now  is  IITV  years,  nearly  ;  but  further  in- 
vestigations are  needed  to  understand  the  true  nature  of  the  phe- 
nomenon. 


ASTRONOMICAL     DRAWINGS.  9 

The  number  of  Sun-spots  does  not  symmetrically  augment  and 
diminish,  but  the  increase  is  more  rapid  than  the  diminution. 

The  period  of  increase  is  only  about  four  years,  while  that  of  de- 
crease is  over  seven  years  ;  each  period  of  Sun-spot  maximum  being 
nearer  the  preceding  period  of  Sun-spot  minimum  than  it  is  to  that 
next  following. 

The  cause  of  these  fluctuations  in  the  solar  energy  is  at  present 
wholly  unknown.  Some  astronomers,  however,  have  attributed  it  to 
the  influence  of  the  planets  Venus  and  Jupiter,  the  period  of  revolution 
of  the  latter  planet  being  not  much  longer  than  the  Sun-spot  period; 
but  this  supposition  lacks  confirmation  from  direct  observations, 
which,  so  far,  do  not  seem  to  be  in  favor  of  the  hypothesis.  At  the 
present  time  the  solar  activity  is  on  the  increase,  and  the  Sun-spots 
will  probably  reach  their  maximum  in  1883.  The  last  minimum 
occurred  in  1879,  when  only  sixteen  small  groups  of  spots  were  ob- 
served during  the  whole  year. 

Sun-spots  vary  in  size  and  appearance  ;  but,  unless  they  are  very 
small,  in  which  case  they  appear  as  simple  black  dots,  they  gener- 
ally consist  of  two  distinct  and  well-characterized  parts,  nearly 
always  present.  There  is  first,  a  central  part,  much  darker  than 
the  other,  and  sharply  divided  from  it,  called  the  "  Umbra ;"  second, 
a  broad,  irregular  radiated  fringe  of  lighter  shade,  completely 
surrounding  the  first,  and  called  the  "Penumbra" 

Reduced  to  its  simplest  expression,  a  Sun-spot  is  a  funnel-shaped 
opening  through  the  chromosphere  and  the  photosphere.  The  inner 
end  of  the  funnel,  or  opening,  gives  the  form  to  the  umbra,  while 
its  sloping  sides  form  the  penumbra. 

The  umbra  of  Sun-spots,  whose  outlines  approximately  follow  the 
irregularities  of  the  penumbral  fringe,  has  a  diameter  which  gener- 
ally exceeds  the  width  of  the  penumbral  ring.  Sometimes  it  appears 
uniformly  black  throughout ;  but  it  is  only  so  by  contrast,  as  is 
proved  when  either  Mercury  or  Venus  passes  near  a  spot  during  a 
transit  over  the  Sun's  disk.  The  umbra  then  appears  grayish,  when 
compared  with  the  jet-black  disk  of  the  planet. 

The  umbra  of  spots  is  rarely  so  simple  as  just  described  ;  but  it 
is  frequently  occupied,  either  partly  or  wholly,  by  grayish  and  rosy 
forms,  somewhat  resem'bling  loosely-entangled  muscular  fibres. 
These  forms  have  been  called  the  Gray  and  Rosy  Veils.  Frequently 
these  veils  appear  as  if  perforated  by  roundish  black  holes,  improp- 
erly called  Nuclei,  which  permit  the  sight  to  penetrate  deeper  into 
the  interior.  To  all  appearance  the  gray  and  rosy  veils  are  of  the 
same  nature  as  the  chromosphere  and  the  faculae,  and  are  therefore 
mainly  composed  of  hydrogen  gas. 


10  THE    TROUVELOT 

Whatever  can  be  known  about  the  interior  of  the  Sun,  must  be 
learned  from  the  observations  of  these  openings,  which  are  compar- 
atively small.  But  whatever  this  interior  may  be,  we  certainly  know 
that  it  is  not  homogeneous.  Apparently,  the  Sun  is  a  gigantic  bub- 
ble, limited  by  a  very  thin  shell.  Below  this  shell  exists  a  large 
open  space  filled  with  invisible  gases,  in  which,  through  the  open- 
ings constituting  the  Sun-spots,  the  gray  and  rosy  veils  described 
above  are  occasionally  seen  floating. 

The  fringe  forming  the  penumbra  of  spots  is  much  more  compli- 
cated than  the  umbra.  In  its  simpler  form,  it  is  composed  of  a  mul- 
titude of  bright,  independent  filaments  of  different  forms  and  sizes, 
partly  projecting  one  above  the  other,  on  the  sloping  wall  of  the 
penumbra,  from  which  they  seem  to  proceed.  Seen  from  the  Earth, 
these  filaments  have  somewhat  the  appearance  of  thatched  straw, 
converging  towards  the  centre  of  the  umbra.  It  is  very  rare,  how- 
ever, that  the  convergence  of  the  penumbral  filaments  is  regular,  and 
great  confusion  sometimes  arises  from  the  entanglement  of  these  fil- 
aments. Some  of  these  elements  appear  straight,  others  are  curved 
or  loop-shaped  ;  while  still  others,  much  larger  and  brighter  than 
the  rest,  give  a  final  touch  to  this  chaos  of  filaments,  from  which 
results  the  general  thatched  and  radiating  appearance  of  the  pe- 
numbra. 

The  extremities  of  the  penumbral  filaments,  especially  of  those 
forming  the  border  of  the  umbra,  are  usually  club-shaped  and  appear 
very  brilliant,  as  if  these  elements  had  been  superheated  by  some 
forces  escaping  through  the  opening  of  the  spots. 

Besides  these  characteristics,  the  Sun-spots  have  others,  which, 
although  not  always  present,  properly  belong  to  them.  Compara- 
tively few  spots  are  so  simple  as  the  form  just  described.  Very  fre- 
quently a  spot  is  accompanied  by  brilliant  faculae,  covering  part  of 
its  umbra  and  penumbra,  and  appearing  to  form  a  part  of  the  spot 
itself. 

When  seen  projected  over  Sun-spots,  the  faculae  appear  intensely 
bright,  and  from  these  peculiarities  they  have  been  called  Luminous 
Bridges.  They  are,  in  fact,  bridges,  but  in  most  cases  they  are  at 
considerable  heights  above  the  spots,  kept  there  by  invisible  forces. 
When  such  spots  with  luminous  bridges  approach  the  Sun's  limb,  it 
is  easy  to  see,  by  the  rapid  apparent  displacement  which  they  under- 
go, that  they  are  above  the  general  level. 

When  the  spots  are  closing  up,  the  inverse  effect  is  sometimes 
observed.  On  several  occasions,  I  have  seen  huge  masses  of  faculae 
advance  slowly  over  the  penumbra  of  a  spot  and  fall  into  the  depths 


A  S  TRONOMICA L    DRA  WINGS.  11 

of  the  umbra,  resembling  gigantic  cataracts.  I  have  seen  narrow 
branches  of  faculae,  which,  after  having  fallen  to  great  depths  in 
the  umbra,  floated  across  it  and  disappeared  under  the  photosphere 
on  the  opposite  side.  I  have  also  seen  luminous  bridges,  resembling 
cables,  tightly  stretched  across  the  spots,  slackening  slowly,  as  if 
loosened  at  one  end,  and  gently  curving  into  the  umbra,  where  they 
formed  immense  loops,  large  enough  to  receive  our  globe. 

It  is  to  be  remarked  that,  in  descending  under-  the  photospheric 
shell,  the  bright  faculae  and  the  luminous  bridges  gradually  lose  their 
brilliancy.  At  first  they  appear  grayish,  but  in  descending  farther 
they  assume  more  and  more  the  pink  color  peculiar  to  the  rosy  veils. 
The  pinkish  color  acquired  by  the  faculae  when  they  reach  a  certain 
depth  under  the  photosphere,  is  precisely  the  color  of  the  chromo- 
sphere and  of  the  solar  protuberances,  as  seen  during  total  eclipses 
of  the  Sun — a  fact  which  furnishes  another  proof  that  the  faculae  are 
of  the  same  nature  as  the  protuberances. 

I  record  here  an  observation  which,  at  first  sight,  may  appeal- 
paradoxical  ;  but  which  seems,  however,  to  be  of  considerable  im- 
portance, as  it  shows  unmistakably  that  the  solar  light  is  mainly,  if 
not  entirely,  generated  on  its  surface,  or  at  least  very  near  to  it. 
On  May  26,  1878,  I  observed  a  large  group  of  Sun-spots  at  a  little 
distance  from  the  east  limb  of  the  Sun.  The  spot  nearest  to  the 
limb  was  partly  covered  over  on  its  eastern  and  western  sides  by 
bright  and  massive  faculae  which  concealed  about  two-thirds  of  the 
whole  spot,  only  a  narrow  opening,  running  from  north  to  south, 
being  left  across  the  middle  of  the  spot.  Owing  to  the  rotundity  of 
the  Sun,  the  penumbra  of  this  spot,  although  partly  covered  by  the 
faculae,  could,  however,  be  seen  on  its  eastern  side,  since  the  sight 
of  the  observer  could  there  penetrate  sidewise  under  the  faculae. 
Upon  that  part  of  the  penumbra  appeared  a  strong  shadow,  rep- 
resenting perfectly  the  outline  of  the  facular  mass  situated  above 
it.  The  phenomenon  was  so  apparent  that  no  error  of  observation 
was  possible,  and  a  good  drawing  of  it  was  secured.  If  this  facula 
had  been  as  bright  beneath  as  it  was  above,  it  is  evident  that  no 
shadow  could  have  been  produced  ;  hence  the  light  of  these  faculae 
must  have  been  mainly  generated  on  or  very  near  their  exterior  sur- 
faces. This,  with  the  well-proved  fact  that  the  bright  faculae  lose 
their  light  in  falling  into  the  interior  of  the  Sun,  seems  to  suggest 
the  idea  that  the  bright  light  emitted  by  the  faculae,  and  very  prob- 
ably all  the  solar  light,  can  be  generated  only  on  its  surface  ;  the 
presence  of  the  coronal  atmosphere  being  perhaps  necessary  to  pro- 
duce it.  Several  times  before  this  observation,  I  had  suspected  that 


12  THE    TROUVELOT 

some  faculae  were  casting  a  shadow,  but  as  this  seemed  so  improba- 
ble, my  attention  was  not  awakened  until  the  phenomenon  became 
so  prominent  that  it  could  not  escape  notice. 

With  due  attention,  some  glimpses  of  the  phenomenon  can  fre- 
quently be  observed  through  the  openings  of  some  of  the  faculae 
projecting  over  the  penumbra  of  Sun-spots.  It  is  very  seldom  that 
the  structure  of  the  penumbra  is  seen  through  such  openings,  which 
usually  appear  as  dark  as  the  umbra  of  the  large  spots,  although 
they  do  not  penetrate  through  the  photosphere  like  the  latter.  It  is 
only  when  the  rents  in  the  faculae  are  numerous  and  quite  large,  that 
the  penumbral  structure  is  recognized  through  them.  Since  these 
superficial  rents  in  the  faculae  do  not  extend  through  the  photo- 
sphere, and  appear  black,  it  seems  evident  that  the  penumbra  seen 
through  them  cannot  be  as  bright  as  it  is  when  no  faculae  are  pro- 
jected upon  it,  and  therefore  that  the  faculae  intercept  light  from  the 
exterior  surface,  which  would  otherwise  reach  the  penumbra. 

While  the  matter  forming  the  faculae  sometimes  falls  into  the 
interior  of  the  Sun,  the  same  kind  of  matter  is  frequently  ejected  in 
enormous  quantities,  and  with  great  force,  from  the  interior,  through 
the  visible  and  invisible  openings  of  the  photosphere,  and  form  the 
protuberances  described  in  the  following  section  of  this  manual  (p. 
20.)  It  is  not  only  the  incandescent  hydrogen  gas  or  the  metallic 
vapors  which  are  thus  ejected,  but  also  cooler  hydrogen  gas,  which 
sometimes  appears  as  dark  clouds  on  the  solar  surface.  On  De- 
cember 12,  1875,  I  observed  such  a  cloud  of  hydrogen  issuing  from 
the  corner  of  a  small  Sun-spot.  It  traveled  several  thousand  miles 
on  the  solar  surface,  in  a  north-easterly  direction,  before  it  became 
invisible. 

Solar  spots  are  formed  in  various  ways  ;  but,  for  the  most  part, 
the  apparition  of  a  spot  is  announced  beforehand,  by  a  great  com- 
motion of  the  solar  surface  at  the  place  of  its  appearance,  and  by 
the  formation  of  large  and  bright  masses  of  faculae,  which  are  usu- 
ally swollen  into  enormous  bubbles  by  the  pressure  of  the  internal 
gases.  These  bubbles  become  visible  in  the  spectroscope  while  they 
are  traversing  the  solar  limb,  as  they  are  then  presented  to  us  side- 
wise.  Under  the  action  of  the  increasing  pressure,  the  base  of  the 
faculae  is  considerably  stretched,  and,  its  weakest  side  finally  giv- 
ing way,  the  facular  mass  is  torn  in  many  places  from  the  solar  sur- 
face, and  is  perforated  by  holes  of  different  sizes  and  forms.  The 
holes  thus  made  along  the  border  of  the  faculae  appear  as  small 
black  spots,  separated  more  or  less  by  the  remaining  portion  of  the 
lacerated  faculae,  and  they  enlarge  more  and  more  at  the  expense  of 


ASTRONOMICAL     DRAWINGS.  13 

the  intervening  portions,  which  thus  become  very  narrow.  This 
perforated  side  of  the  faculse,  offering  less  resistance,  is  gradually 
lifted  up,  as  would  be  the  cover  of  a  box,  for  example,  while  its  op- 
posite side  remains  attached  to  the  surface.  The  facular  matter 
separating  the  small  black  holes  is  greatly  stretched  during  this 
action,  and  forms  long  columns  and  filaments.  These  appear  as 
luminous  bridges  upon  the  large  and  perfectly-formed  spot,  which  is 
then  seen'  under  the  lifted  facular  masses.  The  spots  thus  made 
visible  are  soon  freed  from  the  facular  masses,  which  are  gradually 
shifted  towards  the  opposite  side. 

In  such  cases  the  spots  are  undoubtedly  formed  under  the  faculse 
before  they  can  be  seen.  This  becomes  evident  when  such  spots, 
not  yet  cleared  from  the  faculse  covering  them,  are  observed  near 
the  east  limb  ;  since  in  this  position  the  observer  can  see  through 
the  side-openings  of  the  faculae,  and  sometimes  recognize  the  spots 
under  their  cover. 

It  frequently  occurs  that  the  spots  thus  formed  under  the  faculae 
continue  to  be  partly  covered  by  the  facular  clouds,  the  forces  at 
work  in  them  being  apparently  too  feeble  to  shift  them  aside.  In 
such  cases  these  spots  are  visible  when  they  are  in  the  vicinity  of 
the  limb,  where  they  are  seen  sidewise  ;  but  when  observed  in  the 
east,  in  being  carried  forward  by  the  solar  rotation  towards  the 
centre  of  the  disk,  they  gradually  diminish  in  size,  and  finally  be- 
come invisible.  The  disappearance  of  these  spots,  however,  is  only 
apparent,  being  due  to  the  fact  that,  as  they  advance  towards  the 
centre  of  the  disk,  our  lateral  view  of  them  is  gradually  lost,  and  they 
are  finally  hidden  from  sight  by  the  overhanging  faculae  which  then 
serve  as  a  screen  between  the  observer  and  the  spot.  This  class  of 
spots  may  be  called  Lateral  Spots,  from  the  fact  that  they  can 
only  be  seen  laterally,  and  near  the  Sun's  limb. 

Solar  spots  are  also  formed  in  various  other  ways.  Some,  like 
those  represented  in  Plate  I.,  appearing  without  being  announced 
by  any  apparent  disturbance  of  the  surface,  or  by  the  formation  of 
any  faculae,  form  and  develop  in  a  very  short  time.  Others,  appear- 
ing at  first  as  very  small  spots  having  an  umbra  and  a  penumbra, 
slowly  and  gradually  develop  into  very  large  spots.  This  mode  of 
formation,  which  would  seem  to  be  the  most  natural,  is,  however, 
quite  rare.  Spots  of  this  class  have  a  duration  and  permanence  not 
observed  in  those  of  any  other  type.  These  spots  of  slow  and  regu- 
lar development  are  never  accompanied  by  faculae  or  luminous 
bridges,  nor  have  they  any  gray  or  rosy  veils  in  their  interior  ;  a 
fact  which  may,  perhaps,  account  for  their  permanent  character. 


14  'JHE    TROUVELOT 

Another  class  of  spots,  which  is  also  rare,  appear  as  long  and 
narrow  crevasses  showing  the  penumbral  structure  of  the  ordinary 
spots  ;  but  these  rarely  have  any  umbra.  These  long,  and  sometimes 
exceedingly  narrow  fissures  of  the  solar  envelopes,  with  their  radiated 
penumbral  structure,  strongly  suggest  the  idea  that  the  photosphere 
is  composed  of  a  multitude  of  filamentary  elements  having  the 
granulations  for  summits.  Such  a  crevasse  is  represented  on  Plate 
I.,  and  unites  the  two  spots  which  form  the  group. 

The  duration  of  Sun-spots  varies  greatly.  Some  last  only  for  a 
few  hours  ;  while  others  continue  for  weeks  and  even  months  at  a 
time,  but  not  without  undergoing  changes. 

The  modes  of  disappearance  of  Sun-spots  are  as  various  as  those 
of  their  apparition.  The  spots  rarely  close  up  by  a  gradual  diminu- 
tion or  contraction  of  their  umbra  and  penumbra.  This  mode  of 
disappearance  belongs  exclusively  to  the  spots  deprived  of  faculae 
and  veils.  One  of  the  most  common  modes  of  the  disappearance  of 
a  spot  is  its  invasion  by  large  facular  masses,  which  slowly  advance 
upon  its  penumbra  and  umbra  and  finally  cover  it  entirely.  It  is 
a  process  precisely  the  reverse  of  that  in  which  spots  are  formed  by 
the  shifting  aside  of  the  faculae,  as  above  described.  In  other  types,, 
the  spots  close  up  by  the  gradual  enlargement  of  the  luminous 
bridges  traversing  them,  which  are  slowly  transformed  into  branch- 
es of  the  photosphere,  all  of  the  characteristics  of  which  they  have 
acquired.  In  many  cases,  the  spots  covered  over  by  the  faculae 
continue  to  exist  for  some  time,  hidden  under  these  masses,  as  is  often 
proved,  either  by  the  appearance  of  small  spots  on  the  facular  mass 
left  at  the  place  they  occupied,  or  even  by  the  reappearance  of  the 
same  spot. 

Apart  from  the  general  movement  of  rotation  of  the  solar  sur- 
face, some  of  the  spots  seem  to  be  endowed  with  a  proper  motion  of 
their  own,  which  becomes  greater  the  nearer  the  spots  are  to  the 
solar  equator.  According  to  the  observations  of  Mr.  Carrington, 
the  period  of  rotation  of  the  Sun,  as  deduced  from  the  observations 
of  the  solar  spots  during  a  period  of  seven  years,  is  25  days  at  the 
equator  ;  while  at  50°  of  heliocentric  latitude  it  is  27  days.  But  the 
period  of  rotation,  as  derived  from  the  observations  of  spots  occupy- 
ing the  same  latitude,  is  far  from  being  constant,  as  it  varies  at  dif- 
ferent times,  with  the  frequency  of  the  spots  and  with  the  solar 
activity,  so  that  at  present  the  law  of  these  variations  is  not  well 
known.  From  the  character  of  the  solar  envelope,  it  seems  very 
natural  that  the  rotation  should  differ  in  the  different  zones  and  at 
different  times,  since  this  envelope  is  not  rigid,  but  very  movable,  and 
governed  by  forces  which  are  themselves  very  variable. 


ASTRONOMICAL     DRAWINGS.  15 

Although  it  is  a  general  law  that  the  spots  near  the  equator 
have  a  more  rapid  motion  than  those  situated  in  higher  latitudes, 
yet,  in  many  cases,  the  proper  motion  of  the  spots  is  more  apparent 
than  real.  For  the  most  part,  the  changes  of  form  and  the  rapid 
displacements  observed  in  some  spots  are  only  apparent,  and  due  to 
the  fact  that  the  large  masses  of  faculae  which  are  kept  in  suspense 
above  them  are  very  unstable,  and  change  position  with  the  slight- 
est change  in  the  forces  holding  them  in  suspension.  Since  in 
these  cases  we  view  the  spots  through  the  openings  of  the  faculae 
situated  above  them,  the  slightest  motion  of  these  objects  produces 
an  apparent  motion  in  the  spots,  although  they  have  remained 
motionless.  Accordingly,  it  has  been  remarked  that  of  all  the 
spots,  those  which  have  the  greater  proper  motion  are  precisely 
those  which  have  the  most  faculae  and  luminous  bridges  ;  while  the 
other  spots  in  the  same  regions,  but  not  attended  by  similar  phe- 
nomena, are  comparatively  steady  in  their  movement.  These  last 
spots  are  undoubtedly  better  adapted  than  any  others  to  exhibit  the 
rotation  of  the  Sun  ;  but  it  is  probable  that  this  period  of  rotation 
will  never  be  known  with  accuracy,  simply  because  the  solar  surface 
is  unstable,  and  does  not  rotate  uniformly. 

The  Sun-spots  have  a  remarkable  tendency  to  form  into  groups 
of  various  sizes,  but  whatever  may  be  the  number  of  spots  thus  as- 
sembled, the  group  is  nearly  always  composed  of  two  principal  spots, 
to  which  the  others  are  only  accessories.  The  tendency  of  the  Sun- 
spots  to  assemble  in  pairs  is  general,  and  is  observed  in  all  lati- 
tudes, even  among  the  minute  temporary  groups  formed  in  the  polar 
regions.  Whenever  several  are  situated  quite  close  together,  those 
belonging  to  the  same  group  can  be  easily  recognized  by  this  char- 
acter. Whatever  may  be  the  position  of  the  axis  of  the  two  princi- 
pal spots  of  a  group  when  it  is  first  formed,  this  axis  has  a  decided 
tendency  to  place  itself  parallel  to  the  solar  equator,  no  matter  to 
what  latitude  the  group  belongs  ;  and  if  it  is  disturbed  from  this 
position,  it  soon  returns  to  it  when  the  disturbance  has  ceased. 

It  is  also  remarkable  that  the  spots  observed  at  the  same  time 
remain  in  nearly  the  same  parallel  of  latitude  for  a  greater  or  less 
period  of  time  ;  but  they  keep  changing  their  position  from  year  to 
year,  their  latitude  decreasing  with  the  activity  of  the  solar  forces. 

Among  the  Sun-spots,  those  associated  with  faculae  form  the 
groups  which  attain  the  largest  proportions.  When  such  groups 
acquire  an  apparent  diameter  of  i'  or  more,  they  are  plainly  visible 
to  the  naked  eye,  since  for  a  spot  to  be  visible  to  the  naked  eye 
on  the  Sun,  it  need  only  subtend  an  angle  of  50".  I  have  some- 


16  THE    TROUVELOT 

times  seen  such  groups  through  a  smoky  atmosphere,  when  the  solar 
light  was  so  much  reduced  that  the  disk  could  be  observed  directly 
and  without  injury  to  the  sight. 

The  largest  spot  which  ever  came  under  my  observation  was 
seen  during  the  period  from  the  I3th  to  the  iQth  of  November,  1870. 
This  spot,  which  was  on  the  northern  hemisphere  of  the  Sun,  was 
conspicuous  among  the  smaller  spots  constituting  the  group  to 
which  it  belonged,  and  followed  them  on  the  east.  On  November 
i6th,  when  it  attained  its  largest  size,  the  diameter  of  its  penumbra 
occupied  fully  one-fifth  of  the  diameter  of  the  Sun  ;  its  real  diam- 
eter being,  therefore,  not  less  than  172,000  miles,  or  nearly  22  times 
the  diameter  of  the  Earth.  As  the  umbra  of  this  spot  occupied  a 
little  more  than  one-third  of  its  whole  diameter,  seven  globes  like 
our  own,  placed  side  by  side  on  a  straight  line,  could  easily  have 
passed  through  this  immense  gap.  To  fill  the  area  of  this  opening, 
about  45  such  globes  would  have  been  needed.  This  spot  was,  of 
course,  very  easily  seen  with  the  naked  eye,  its  diameter  being 
almost  eight  times  that  required  for  a  spot  to  be  visible  without  a 
telescope. 

Ancient  historians  often  speak  of  obscurations  of  the  Sun,  and  it 
has  been  supposed  by  some  astronomers  that  this  phenomenon 
might  have  been  due  in  some  cases  to  the  apparition  of  large  spots. 
A  few  spots  on  the  surface  of  the  Sun,  like  that  just  described, 
would  sensibly  reduce  its  light. 

Besides  the  ordinary  Sun-spots  already  described,  others  are  at 
times  observed  on  the  surface  of  the  Sun,  which  show  some  of  the 
same  characteristics,  but  never  attain  so  large  proportions.  They 
always  appear  as  if  seen  through  a  fog,  or  veil,  between  the 
granulations  of  the  solar  surface.  On  account  of  their  vagueness  and 
ill-defined  contours,  I  have  proposed  for  these  objects  the  term, 
"  Veiled  Spots.  "  Veiled  spots  have  a  shorter  duration  than  the 
ordinary  spots,  the  smaller  types  sometimes  forming  and  vanishing 
in  a  few  minutes.  Some  of  the  larger  veiled  spots,  however,  re- 
main visible  for  several  days  in  succession,  and  show  the  charac- 
teristics of  other  spots  in  regard  to  the  arrangement  of  their  parts. 
The  veiled  spots  have  no  umbra  or  penumbra,  although  they 
are  usually  accompanied  by  faculae  resembling  those  seen  near  the 
ordinary  spots.  They  are  frequently  seen  in  the  polar  regions,  but 
are  there  always  of  small  size  and  of  short  duration.  The  veiled 
spots  are  larger,  and  more  apt  to  arrange  themselves  into  groups, 
in  the  regions  occupied  by  the  ordinary  spots,  and  it  is  not  rare  to 
observe  such  spots  transform  themselves  into  ordinary  spots,  and 
vice  versa.  The  veiled  spots,  therefore,  seem  to  be  ordinary  spots 


ASTRONOMICAL    DRAWINGS.  17 

filled  up,  or  covered  over  by  the  granulations  and  semi-transparent 
gases  composing  the  chromospheric  layer.  That  it  is  so,  becomes 
more  evident,  from  the  fact  that  large  Sun-spots  in  process  of  dimi- 
nution are  sometimes  gradually  covered  with  faint  and  scattered 
granules  which  descend  in  long,  narrow  filaments,  and  become 
less  and  less  distinguishable  as  they  attain  greater  depth.  This 
phenomenon,  associated  with  the  fact  that  the  luminous  bridges  seen 
over  the  Sun-spots  which  are  closing  up  are  sometimes  transformed 
into  branches  which  show  the  characteristic  structure  of  the  photo- 
sphere, goes  far  to  prove  that  the  solar  envelopes  are  mainly  com- 
posed of  an  innumerable  quantity  of  radial  filaments  of  varying  height. 

The  group  of  Sun-spots  represented  in  Plate  I.,  was  observed 
and  drawn  on  June  i;th,  1875,  at  ;h.  3Om.  A.  M.  The  first  traces  of 
this  group  were  seen  on  June  I5th,  at  noon,  and  consisted  of  three 
small  black  dots  disseminated  among  the  granulations.  At  that 
time,  no  disturbance  of  the  surface  was  noticeable,  and  no  faculae  were 
seen  in  the  vicinity  of  these  spots.  On  June  i6th,  at  8  o'clock  A.M., 
the  three  small  spots  had  become  considerably  enlarged,  and,  as 
usual,  the  group  consisted  of  two  principal  spots.  Between  these  two 
spots  all  was  in  motion:  the  granulations,  stretched  into  long,  wavy, 
parallel  lines,  had  somewhat  the  appearance  of  a  liquid  in  rapid 
motion.  At  I  o'clock,  P.  M.,  on  the  same  day,  the  group  had  con- 
siderably enlarged;  the  faculae,  the  granulations,  and  the  penumbral 
filaments  being  interwoven  in  an  indescribable  manner.  On  the 
morning  of  the  I7th,  these  spots  had  assumed  the  complicated  form 
and  development  represented  in  the  drawing  ;  while  at  the  same  time 
two  conspicuous  veiled  spots  were  seen  on  the  left  hand,  at  some 
distance  above  the  group. 

Some  luminous  bridges  are  visible  upon  the  left  hand  spot,  trav- 
ersing the  penumbra  and  umbra  of  this  spot  in  various  directions. 
The  umbra  of  one  of  the  spots  is  occupied,  and  partly  filled  with 
gray  and  rosy  veils,  similar  to  those  above  described,  and  the  granu- 
lations of  the  solar  surface  form  a  background  to  the  group  of  spots. 

This  group  of  spots  was  not  so  remarkable  for  its  size  as  for  its 
complicated  structure.  The  diameter  of  the  group  from  east  to  west 
was  only  2%  minutes  of  arc,  or  about  67,000  miles.  The  upper 
part  of  the  umbra  of  the  spot  situated  on  the  right  hand  side  of  the 
group  was  nearly  7,000  miles  in  diameter,  or  less  by  1,000  miles  than 
the  diameter  of  the  Earth.  Some  of  the  long  filaments  composing 
that  part  of  the  penumbra,  situated  on  the  left  hand  side  of  the  same 
spot,  were  17,000  miles  in  length.  One  of  these  fiery  elements  would 
be  sufficient  to  encircle  two-thirds  of  the  circumference  of  the  Earth. 


18  THE    TROUVELOT 


SOLAR  PROTUBERANCES. 
PLATE  II. 

THE  chromosphere  forming  the  outlying  envelope  of  the  Sun,  is 
subject,  as  has  been  shown  above,  to  great  disturbances  in  cer- 
tain regions,  causing  considerable  upheavals  of  its  surface  and  violent 
outbursts  of  its  gases.  From  these  upheavals  and  outbursts  of  the 
chromosphere  result  certain  curious  and  very  interesting  forms,  which 
are  known  under  the  name  of  "  Solar  Protuberances"  "Prominences" 
or  "  Flame  sT 

These  singular  forms,  which  could,  until  recently,  be  observed 
only  during  the  short  duration  of  the  total  eclipses  of  the  Sun,  can 
now  be  seen  on  every  clear  day  with  the  spectroscope,  thanks  to 
Messrs.  Janssen  and  Lockyer,  to  whose  researches  solar  physics  is 
so  much  indebted. 

The  solar  protuberances,  the  Sun-spots,  and  the  faculae  to  which 
they  are  closely  related,  are  confined  within  the  same  general 
regions  of  the  Sun,  although  the  protuberances  attain  higher  helio- 
centric latitudes. 

There  is  certainly  a  very  close  relation  between  the  faculae  and 
the  solar  protuberances,  since  when  a  group  of  the  faculae  traverses 
the  Sun's  limb,  protuberances  are  always  seen  at  the  same  place.  It 
seems  very  probable  that  the  faculae  and  the  protuberances  are  in 
the  main  identical.  The  faculae  may  be  the  brighter  portion  of  the 
protuberances,  consisting  of  gases  which  are  still  undergoing  a  high 
temperature  and  pressure;  while  the  gases  which  have  been  re- 
lieved from  this  pressure  and  have  lost  a  considerable  amount  of 
their  heat,  may  form  that  part  of  the  protuberances  which  is  only 
visible  on  the  Sun's  limb. 

A  daily  study  of  the  solar  protuberances,  continued  for  ten  years, 
has  shown  me  that  these  objects  are  distributed  on  two  zones  which 
are  equidistant  from  the  solar  equator,  and  parallel  with  it.  The 
zone  arrangement  of  the  protuberances  is  more  easily  recognized 
during  the  years  of  minimum  solar  activity,  as  in  these  years  the 


ASTRONOMICAL     DRAWINGS.  19 

zones  are  very  narrow  and  widely  separated.  During  these  years 
the  belt  of  protuberances  is  situated  between  40°  and  45°  of  latitude, 
north  and  south.  In  years  of  great  solar  activity  the  zones  spread 
considerably  on  either  side  of  these  limits,  especially  towards  the 
equator,  which  they  nearly  reach,  only  a  narrow  belt,  usually  free 
from  protuberances,  remaining  between  them.  Towards  the  poles 
the  zones  do  not  spread  so  much,  and  there  the  space  free  from  pro- 
tuberances is  considerably  greater  than  it  is  at  the  equator. 

During  years  of  maximum  solar  activity,  the  protuberances,  like 
the  Sun-spots  and  the  faculae,  are  very  numerous,  very  large,  and 
very  complicated — sometimes  occupying  a  great  part  of  the  whole 
solar  limb.  As  many  as  twenty  distinct  flames  are  sometimes  ob- 
served at  one  time.  In  years  of  minimum  solar  activity,  on  the  con- 
trary, the  prominences  are  very  few  in  number,  and  they  are  of 
small  size;  but,  as  far  as  my  observations  go,  they  are  never  totally 
absent. 

In  general,  the  solar  flames  undergo  rapid  changes,  especially 
those  which  are  situated  in  the  vicinity  of  Sun-spots,  although  they 
occasionally  remain  unchanged  in  appearance  and  form  for  several 
hours  at  a  time.  The  protuberances  situated  in  higher  latitudes  are 
less  liable  to  great  and  sudden  changes,  often  retaining  the  same 
form  for  several  days.  The  changes  observed  in  the  protuberances 
of  the  equatorial  regions  are  due  in  part  to  the  comparatively  great 
changes  in  their  position  with  respect  to  the  spectator,  which  are 
occasioned  by  the  rotation  of  the  Sun.  This  rotation,  of  course,  has 
a  greater  angular  velocity  on  the  equator  than  in  higher  latitudes. 
In  most  cases,  however,  the  changes  of  the  equatorial  protuberances 
are  too  great  and  too  sudden  to  be  thus  explained.  They  are,  in 
fact,  due  to  the  greater  solar  activity  developed  in  the  equatorial 
zones,  and  wherever  spots  are  most  numerous. 

The  solar  protuberances  appear  under  various  shapes,  and  are 
often  so  complicated  in  appearance  that  they  defy  description.  Some 
resemble  huge  clumsy  masses  having  a  few  perforations  on  their 
sides  ;  while  others  form  a  succession  of  arches  supported  by  pillars 
of  different  styles.  Others  form  vertical  or  inclined  columns,  often 
surmounted  by  cloud-like  masses,  or  by  various  appendages,  which 
sometimes  droop  gracefully,  resembling  gigantic  palm  leaves.  Some 
resemble  flames  driven  by  the  wind  ;  others,  which  are  composed  of 
a  multitude  of  long  and  narrow  filaments,  appear  as  immense  fiery 
bundles,  from  which  sometimes  issue  long  and  delicate  columns  sur- 
mounted by  torch-like  objects  of  the  most  fantastic  pattern.  Some 
others  resemble  trees,  or  animal  forms,  in  a  very  striking  manner ; 


20  THE    TROUVELOT 

while  still  others,  apparently  detached  from  the  solar  limb,  float 
above  it,  forming  graceful  streamers  or  clouds  of  various  shapes. 
Some  of  the  protuberances  are  very  massive,  while  others  are  so 
thin  and  transparent  as  to  form  a  mere  veil,  through  which  more 
distant  flames  can  easily  be  seen. 

Notwithstanding  this  variety  of  form,  two  principal  classes  of 
solar  protuberances  may  be  recognized  :  the  cloud-like  or  quiescent, 
and  the  eruptive  or  metallic  protuberances. 

The  first  class,  which  is  the  most  common,  comprises  all  the 
cloud-like  protuberances  resting  upon  the  chromosphere  or  floating 
about  it.  The  protuberances  of  this  type  often  obtain  enormous 
horizontal  proportions,  and  it  is  not  rare  to  see  some  among  them 
occupying  20°  and  30°  of  the  solar  limb.  The  height  attained  by 
protuberances  of  this  class  does  not  correspond  in  general  to  their 
longitudinal  extent ;  although  some  of  their  branches  attain  con- 
siderable elevations.  These  prominences  very  seldom  have  the  bril- 
liancy displayed  by  the  other  type,  and  are  sometimes  so  faint  as  to 
be  seen  with  difficulty.  Although  it  is  generally  stated  by  observ- 
ers that  some  of  the  protuberances  belonging  to  this  class  are 
detached  from  the  solar  surface,  and  kept  in  suspension  above  the 
surface,  like  the  clouds  in  our  atmosphere,  yet  it  seems  to  me  very 
doubtful  whether  protuberances  are  ever  disconnected  from  the  chro- 
mosphere, since,  in  an  experience  of  ten  years,  I  have  never  been 
able  to  satisfy  myself  that  such  a  thing  has  occurrred.  Many  of 
them  have  appeared  to  me  at  first  sight  to  be  detached  from  the  sur- 
face, but  with  a  little  patience  and  attention  I  was  always  able  to 
detect  faint  traces  of  filamentary  elements  connecting  them  with  the 
chromosphere.  Quite  often  I  have  seen  bright  protuberances  gradu- 
ally lose  their  light  and  become  invisible,  while  soon  after  they  had 
regained  it,  and  were  as  clearly  visible  as  before.  Observations  of  this 
kind  seem  to  show  that  while  the  prominences  are  for  the  most  part 
luminous,  there  are  also  a  few  which  are  non-luminous  and  invisible 
to  the  eye.  These  dark  and  invisible  forms  are  most  generally 
found  in  the  vicinity  of  Sun-spots  in  great  activity.  When  observ- 
ing such  regions  with  the  spectroscope,  it  is  not  rare  to  encounter 
them  in  the  form  of  large  dark  spots  projecting  on  the  solar  spectrum 
near  the  hydrogen  lines.  On  July  28th,  1872,  I  observed  with  the 
spectroscope  a  dark  spot  of  this  kind  issuing  from  the  vicinity  of  a 
large  Sun-spot,  and  extending  over  one-fifth  of  the  diameter  of  the 
Sun.  This  object  had  been  independently  observed  in  France  a  lit- 
tle earlier  by  M.  Chacornac  with  the  telescope,  in  which  it  appeared 
as  a  bluish  streak. 


ASTRONOMICAL     DRAWINGS.  21 

The  second  class  of  solar  protuberances,  comprising  the  eruptive 
type,  is  the  most  interesting,  inasmuch  as  it  conveys  to  us  a  concep- 
tion of  the  magnitude  and  violence  of  the  solar  forces.  The  protu- 
berances of  this  class,  which  are  always  intensely  bright,  appear  for 
the  most  part  in  the  immediate  vicinity  of  Sun-spots  or  faculae. 
These  protuberances,  which  seem  to  be  due  to  the  outburst  of  the 
chromosphere,  and  to  the  violent  ejection  of  incandescent  gases  and 
metallic  vapors  from  the  interior  of  the  Sun,  sometimes  attain  gigan- 
tic proportions  and  enormous  heights. 

While  the  spectrum  of  the  protuberances  of  the  cloudy  type  is 
simple,  and  usually  composed  of  four  hydrogen  lines  and  the  yellow 
line  D3,  that  of  the  eruptive  class  is  very  complicated,  and,  besides 
the  hydrogen  lines  and  D3,  it  often  exhibits  the  bright  lines  of  so- 
dium, magnesium,  barium,  titanium,  and  iron,  and  occasionally,  also, 
a  number  of  other  bright  lines. 

The  phenomena  of  a  solar  outburst  are  grand  and  imposing. 
Suddenly  immense  and  acute  tongues  and  jets  of  flames  of  a  daz- 
zling brilliancy  rise  up  from  the  solar  limb  and  extend  in  various 
directions.  Some  of  these  fiery  jets  appear  perfectly  rigid,  and 
remain  apparently  motionless  in  the  midst  of  the  greatest  disorder. 
Immense  straps  and  columns  form  and  rise  in  an  instant,  bending 
and  waving  in  all  sorts  of  ways  and  assuming  innumerable  shapes. 
Sometimes  powerful  jets  resembling  molten  metal  spring  up  from 
the  Sun,  describing  graceful  parabolas,  while  in  their  descent  they 
form  numerous  fiery  drops  which  acquire  a  dazzling  brilliancy  when 
they  approach  the  surface. 

The  upward  motion  of  the  protuberances  in  process  of  formation 
is  sometimes  very  rapid.  Some  protuberances  have  been  observed 
to  ascend  in  the  solar  atmosphere  at  the  rate  of  from  120  to  497 
miles  a  second.  Great  as  this  velocity  may  appear,  it  is  nevertheless 
insignificant  when  compared  with  that  sometimes  attained  by  protu- 
berances moving  in  the  line  of  sight  instead  of  directly  upwrards. 
Movements  of  this  kind  are  indicated  by  the  displacement  of  the 
bright  or  dark  lines  in  the  spectrum.  A  remarkable  instance  of  this 
kind  occurred  on  the  26th  of  June,  1874.  On  that  day  I  observed  a 
displacement  of  the  hydrogen  C  line  corresponding  to  a  velocity  of 
motion  of  1,600  miles  per  second.  The  mass  of  hydrogen  gas  in 
motion  producing  such  a  displacement  was,  according  to  theory, 
moving  towards  the  Earth  at  this  incredible  rate,  when  it  instantly 
vanished  from  sight  as  if  it  had  been  annihilated,  and  was  seen  no 
more. 

Until  recently  the  protuberances  had  not  been  observed  to  rise 


22  THE    TROUVELOT 

more  than  200,000  miles  above  the  solar  surface;  but,  on  October  /thr 
1880,  a  flame,  which  had  an  elevation  of  80,000  miles  when  I  observed 
it  at  8h.  55m.  A.  M.,  had  attained  the  enormous  altitude  of  350,000 
miles  when  it  was  observed  at  noon  by  Professor  C.  A.  Young.  If 
we  had  such  a  protuberance  on  the  Earth,  its  summit  would  be  at  a 
height  sufficient  not  merely  to  reach,  but  to  extend  100,000  miles 
beyond  the  Moon. 

Although  the  solar  protuberances  represented  in  Plate  II.  have 
not  the  enormous  proportions  attained  by  some  of  these  objects,  yet 
they  are  as  characteristic  as  any  of  the  largest  ones,  and  afford  a 
good  illustration  of  the  purely  eruptive  type  of  protuberances.  The 
height  of  the  largest  column  in  the  group  equals  4'  43",  or  a  little 
over  126,000  miles.  A  large  group  of  Sun-spots  was  in  the  vicinity 
of  these  protuberances  when  they  were  observed  and  delineated. 


ASTRONOMICAL     DRA  WINGS.  23 


TOTAL  ECLIPSE  OF  THE  SUN. 
PLATE  III. 

A  SOLAR  eclipse  is  due  to  the  passage  of  the  Moon  directly  be- 
tween the  observer  and  the  Sun.  Such  an  eclipse  can  only  occur  at 
New  Moon,  since  it  is  only  at  that  time  that  our  satellite  passes  be- 
tween us  and  the  Sun.  The  Moon's  orbit  does  not  lie  precisely  in  the 
same  plane  as  the  orbit  of  the  Earth,  but  is  inclined  about  five  de- 
grees to  it,  otherwise  an  eclipse  of  the  Sun  would  occur  at  every 
New  Moon,  and  an  eclipse  of  the  Moon  at  every  Full  Moon. 

Since  the  Moon's  orbit  is  inclined  to  that  of  the  Earth,  it  must 
necessarily  intersect  this  orbit  at  two  opposite  points.  These  points 
are  called  the  nodes  of  the  Moon's  orbit.  When  our  satellite  passes 
through  either  of  the  nodes  when  the  Moon  is  new,  it  appears  inter- 
posed to  some  extent  between  the  Sun  and  the  Earth,  and  so  pro- 
duces a  solar  eclipse  ;  while  if  it  passes  a  node  when  the  Moon  is  full, 
it  is  more  or  less  obscured  by  the  Earth's  shadow,  which  then  pro- 
duces an  eclipse  of  the  Moon.  But,  on  the  other  hand,  when  the 
New  Moon  and  the  Full  Moon  do  not  coincide  with  the  passage  of 
our  satellite  through  the  nodes  of  its  orbit,  no  eclipse  can  occur, 
since  the  Moon  is  not  then  on  a  line  with  the  Sun  and  the  Earth, 
but  above  or  below  that  line. 

Owing  to  the  ellipticity  of  the  Moon's  orbit,  the  distance  of  our 
satellite  from  the  Earth  varies  considerably  during  each  of  its  revo- 
lutions around  us,  and  its  apparent  diameter  is  necessarily  subject  to 
corresponding  changes.  Sometimes  it  is  greater,  sometimes  it  is  less, 
than  the  apparent  diameter  of  the  Sun.  If  it  is  greater  at  the  time 
of  a  solar  eclipse,  the  eclipse  will  be  total  to  a  terrestrial  observer 
stationed  nearly  on  the  line  of  the  centres  of  the  Sun  and  Moon, 
while  it  will  be  only  partial  to  another  observer  stationed  further 
from  this  line.  But  the  Moon's  distance  from  the  Earth  may  be  so 
great  and  its  apparent  diameter  consequently  so  small  that  even 
those  observers  nearest  the  central  line  of  the  eclipse  see  the  bor- 
der of  the  Sun  all  round  the  black  disk  of  the  Moon  ;  the  eclipse  is 


24  THE     TROUVELOT 

then  annular.  Even  during  the  progress  of  one  and  the  same  eclipse 
the  distance  of  the  Moon  from  the  parts  of  the  Earth  towards  which 
its  shadow  is  directed  may  vary  so  much  that,  while  the  eclipse  is 
total  to  some  observers,  others  equally  near  the  central  line,  but  sta- 
tioned at  a  different  place,  will  see  it  as  annular. 

The  shadow  cast  by  the  Moon  on  the  Earth  during  total  eclipses, 
travels  along  upon  the  surface  of  the  Earth,  in  consequence  of  the 
daily  movement  of  rotation  of  our  globe  combined  with  the  move- 
ments of  the  Earth  and  Moon  in  their  orbits.  The  track  of  the 
Moon's  shadow  over  the  Earth's  surface  has  a  general  eastward 
course,  so  that  the  more  westerly  observers  see  it  earlier  than  those 
east  of  them.  An  eclipse  may  continue  total  at  one  place  for  nearly 
eight  minutes,  but  in  ordinary  cases  the  total  phase  is  much  shorter. 

The  nodes  of  the  Moon's  orbit  do  not  invariably  occupy  the 
same  position,  but  move  nearly  uniformly,  their  position  with  regard 
to  the  Sun,  Earth,  and  Moon  being  at  any  time  approximately  what 
it  formerly  was  at  a  series  of  times  separated  by  equal  intervals 
from  each  other.  Each  interval  comprises  223  lunations,  or  18 
years,  1 1  days,  and  7  or  8  hours.  The  eclipses  which  occur  within 
this  interval  are  almost  exactly  repeated  during  the  next  similar 
interval.  This  period,  called  the  "  Saros,"  was  well  known  to  the 
ancients,  who  were  enabled  by  its  means  to  predict  eclipses  with 
some  certainty. 

A  total  eclipse  of  the  Sun  is  a  most  beautiful  and  imposing 
phenomenon.  At  the  predicted  time  the  perfectly  round  disk  of  the 
Sun  becomes  slightly  indented  at  its  western  limb  by  the  yet  invisi- 
ble Moon.  This  phenomenon  is  known  as  the  "  first  contact." 

The  slight  indentation  observed  gradually  increases  with  the  ad- 
vance of  the  Moon  from  west  to  east,  the  irregularities  of  the  sur- 
face of  our  satellite  being  plainly  visible  on  the  border  of  the  dark 
segment  advancing  on  the  Sun's  disk.  With  the  advance  of  the  Moon 
on  the  Sun,  the  light  gradually  diminishes  on  the  Earth.  Every 
object  puts  on  a  dull  and  gloomy  appearance,  as  when  night  is  ap- 
proaching; while  the  bright  sky,  losing  its  light,  changes  its  pure 
azure  for  a  livid  grayish  color. 

Two  or  three  minutes  before  totality  begins,  the  solar  crescent, 
reduced  to  minute  proportions,  gives  comparatively  so  little  light 
that  faint  traces  of  the  Sun's  atmosphere  appear  on  the  western  side 
behind  the  dark  body  of  the  Moon,  whose  limb  then  becomes  visible 
outside  of  the  Sun.  I  observed  this  phenomenon  at  Creston  during 
the  eclipse  of  1878.  From  15  to  20  seconds  before  totality,  the  nar- 
row arc  of  the  Sun's  disk  not  yet  obscured  by  the  Moon  seems  to 


ASTRONOMICAL     DRAWINGS.  25 

break  and  separate  towards  the  extremities  of  its  cusps,  which,  thus 
divided,  form  independent  points  of  light,  which  are  called  "Bailys 
beads''  A  moment  after,  the  whole  solar  crescent  breaks  into  nu- 
merous beads  of  light,  separated  by  dark  intervals,  and,  suddenly, 
they  all  vanish  with  the  last  ray  of  Sunlight,  and  totality  has  begun 
with  the  "  second  contact."  This  phenomenon  of  Baily's  beads  is 
undoubtedly  caused  by  the  irregularities  of  the  Moon's  border,  which, 
on  reaching  the  solar  limb,  divide  the  thin  solar  crescent  into  as 
many  beads  of  light  and  dark  intervals  as  there  are  peaks  and  ravines 
seen  sidewise  on  that  part  of  the  Moon's  limb. 

With  the  disappearance  of  the  last  ray  of  light,  the  planets  and 
the  stars  of  the  first  and  second  magnitude  seem  to  light  up  and  be- 
come visible  in  the  sky.  The  darkness,  which  had  been  gradually 
creeping  in  with  the  progress  of  the  eclipse,  is  then  at  its  maximum. 
Although  subject  to  great  variations  in  different  eclipses,  the  dark- 
ness is  never  so  great  as  might  be  expected  from  the  complete  ob- 
scuration of  our  luminary,  as  the  part  of  our  atmosphere  which  is 
still  exposed  to  the  direct  rays  of  the  Sun,  reflects  to  us  some  of  that 
light,  which  thus  diminishes  the  darkness  resulting  from  the  disap- 
pearance of  the  Sun.  Usually  the  darkness  is  sufficient  to  prevent 
the  reading  of  common  print,  and  to  deceive  animals,  causing  them 
to  act  as  if  night  was  really  approaching.  During  totality  the  tem- 
perature decreases,  while  the  humidity  of  the  atmosphere  augments. 

Simultaneously  with  the  disappearance  of  Baily's  beads,  a  pale, 
soft,  silvery  light  bursts  forth  from  behind  the  Moon,  as  if  the  Sun, 
in  disappearing,  had  been  vaporized  and  expanded  in  all  directions 
into  soft  phosphorescent  rays  and  streamers.  This  pale  light  is 
emitted  by  gases  constituting  the  solar  atmosphere  surrounding  the 
bright  nucleus  now  obscured  by  the  dark  body  of  our  satellite.  This 
solar  atmosphere  is  called  Corona,  from  its  distant  resemblance  to 
the  aureola,  or  glory,  represented  by  ancient  painters  around  the 
heads  of  saints. 

With  the  bursting  forth  of  the  corona,  a  very  thin  arc  of  bright 
white  light  is  seen  along  the  Moon's  limb,  where  the  solar  crescent 
has  just  disappeared.  This  thin  arc  of  light  is  the  reversing  layer, 
which,  when  observed  with  the  spectroscope  at  that  moment,  exhib- 
its bright  lines  answering  to  the  dark  lines  of  the  ordinary  solar 
spectrum.  Immediately  above  this  reversing  layer,  and  concentric 
with  it,  appears  the  pink-colored  chromospheric  layer,  with  its  curi- 
ously shaped  flames  and  protuberances.  During  totality,  the  chro- 
mosphere and  protuberances  are  seen  without  the  aid  of  the  spec- 
troscope, and  appear  of  their  natural  color,  which,  although  some- 


26  THE    TROUVELOT 

what  varying  in  their  different  parts,  is,  on  the  whole,  pinkish,  and 
similar  to  that  of  peach-blossoms;  yet  it  is  mixed  here  and  there 
with  delicate  prismatic  hues,  among  which  the  pink  and  straw 
colors  predominate. 

The  color  of  the  corona  seems  to  vary  in  every  eclipse,  but  as  its 
tints  are  very  delicate,  it  may  depend,  in  a  great  measure,  upon  the 
vision  of  the  observer  ;  although  there  seems  to  be  no  doubt  that 
there  are  real  variations.  At  Creston,  in  1878,  it  appeared  to  both 
Professor  W.  Harkness  and  myself  of  a  decided  pale  greenish  hue. 

The  corona  appears  under  different  forms,  and  has  never  been 
observed  twice  alike.  Its  dimensions  are  also  subject  to  consid- 
erable variations.  Sometimes  it  appears  regular  and  very  little 
extended,  its  distribution  around  the  Sun  being  almost  uniform; 
although  in  general  it  spreads  a  little  more  in  the  direction  of  the 
ecliptic,  or  of  the  solar  equator.  At  other  times  it  appears  much 
larger  and  more  complicated,  and  forms  various  wings  and  append- 
ages, which  in  some  cases,  as  in  1878,  extend  to  immense  distances; 
while  delicate  rays  radiate  in  straight  or  curved  lines  from  the  spaces 
left  in  the  polar  regions  between  the  wings.  The  corona  has  some- 
times appeared  as  if  divided  by  immense  dark  gaps,  apparently  free 
from  luminous  matter,  and  strongly  resembling  the  dark  rifts  seen 
in  the  tails  of  comets.  This  was  observed  in  Spain  and  Sicily  during 
the  total  eclipse  of  the  Sun  in  1870.  Different  structures,  forming 
wisps  and  streamers  of  great  length,  and  interlaced  in  various  ways, 
are  sometimes  present  in  the  corona,  while  faint  but  more  compli- 
cated forms,  distantly  resembling  enormous  solar  protuberances 
with  bright  nuclei,  have  also  been  observed. 

As  the  Moon  continues  its  eastward  progress,  it  gradually  covers 
the  chromosphere  and  the  solar  protuberances  on  the  eastern  side  of 
the  Sun;  while,  at  the  same  time,  the  protuberances  and  the  chro- 
mosphere on  the  opposite  limb  gradually  appear  from  under  the  re- 
treating Moon.  Then,  the  thin  arc  of  the  reversing  layer  is  visible 
for  an  instant,  and  is  instantly  followed  by  the  appearance  of  a  point 
of  dazzling  white  light,  succeeded  immediately  by  the  apparition  of 
Baily's  beads  on  each  side,  and  totality  is  over,  with  this  third  con- 
tact. The  corona  continues  to  be  visible  on  the  eastern  side  of  the 
Sun  for  several  minutes  longer,  and  then  rapidly  vanishes. 

The  thin  solar  crescent  increases  in  breadth  as  the  Moon  ad- 
vances; while,  at  the  same  time,  the  darkness  and  gloom  spread  over 
nature  gradually  disappear,  and  terrestrial  objects  begin  to  resume 
their  natural  appearance.  Finally  the  limb  of  the  Moon  separates 
from  that  of  the  Sun  at  the  instant  of  "  fourth  contact,"  and  the 
eclipse  is  over. 


ASTRONOMICAL    DRAWINGS.  27 

The  phenomena  exhibited  by  the  corona  in  different  eclipses  are 
very  complex,  and,  so  far,  they  have  not  been  sufficiently  studied  to 
enable  us  to  understand  the  true  nature  of  the  solar  atmosphere. 
From  the  spectral  analysis  of  the  corona,  and  the  phenomena  of 
polarization,  it  has  been  learned,  at  least,  that  while  the  matter 
composing  the  upper  part  of  the  solar  atmosphere  is  chiefly  com- 
posed of  an  unknown  substance,  producing  the  green  line  1474,  its 
lower  part  is  mainly  composed  of  hydrogen  gas  at  different  temper- 
atures, a  part  of  which  is  self-luminous,  while  the  other  part  only 
reflects  the  solar  light.  But  the  proportion  of  the  gaseous  particles 
emitting  light,  to  those  simply  reflecting  it,  is  subject  to  con- 
siderable variations  in  different  eclipses.  At  present  it  would  seem 
that  in  years  of  great  solar  disturbances,  the  particles  emitting  light 
are  found  in  greater  quantity  in  the  corona  than  those  reflecting 
it;  but  further  observations  will  be  required  to  confirm  these  views. 

It  is  very  difficult  to  understand  how  the  corona,  which  in  cer- 
tain eclipses  extends  only  one  diameter  of  the  Sun,  should,  in  other 
cases,  as  in  1878,  extend  to  the  enormous  distance  of  twelve  times 
the  same  diameter.  Changes  of  such  magnitude  in  the  solar  atmo- 
sphere, if  due  to  the  operation  of  forces  with  which  we  are  acquaint- 
ed, cannot  yet  be  accounted  for  by  what  is  known  of  such  forces. 
Their  causes  are  still  as  mysterious  as  those  concerned  in  the  pro- 
duction of  the  monstrous  tails  displayed  by  some  comets  on  their 
approach  to  the  Sun. 

Plate  3,  representing  the  total  eclipse  of  the  Sun  of  July  29th, 
1878,  was  drawn  from  my  observations  made  at  Creston,  Wyoming 
Territory,  for  the  Naval  Observatory.  The  eclipse  is  represented  as 
seen  in  a  refracting  telescope,  having  an  aperture  of  6y$  inches,  and 
as  "it  appeared  a  few  seconds  before  totality  was  over,  and  when  the 
chromosphere  was  visible  on  the  western  limb  of  the  Sun.  The  two 
long  wings  seen  on  the  east  and  west  side  of  the  Sun,  appeared  con- 
siderably larger  in  the  sky  than  they  are  represented  in  the  picture. 


28  THE     TROUVELOT 


THE   AURORA  BOREALIS. 
PLATE  IV. 

THE  name  of  Polar  Auroras  is  given  to  certain  very  remarkable 
luminous  meteoric  phenomena  which  appear  at  intervals  above  the 
northern  or  the  southern  horizons  of  both  hemispheres  of  the  Earth. 
When  the  phenomenon  is  produced  in  our  northern  sky,  it  is  called 
"Aurora  Borealis,"  or  "  Northern  Lights;"  and  when  it  appears  in 
the  southern  sky,  it  is  called  "  Aurora  Australis,"  or  southern 
aurora. 

Marked  differences  appear  in  the  various  auroras  observed  from 
our  northern  latitudes.  While  some  simply  consist  in  a  pale,  faint 
luminosity,  hardly  distinguishable  from  twilight,  others  present  the 
most  gorgeous  and  remarkable  effects  of  brightness  and  colors. 

A  great  aurora  is  usually  indicated  in  the  evening  soon  after 
twilight,  by  a  peculiar  grayish  appearance  of  the  northern  sky  just 
above  the  horizon.  The  grayish  vapors  giving  that  appearance,  con- 
tinuing to  form  there,  soon  assume  a  dark  and  gloomy  aspect,  while 
they  gradually  take  the  form  of  a  segment  of  a  circle  resting  on  the 
horizon.  At  the  same  time  that  this  dark  segment  is  forming,  a 
soft  pearly  light,  which  seems  to  issue  from  its  border,  spreads  up  in 
the  sky,  where  it  gradually  vanishes,  being  the  brightest  at  its  base. 
This  arc  of  light,  gradually  increasing  in  extent  as  well  as  in  bright- 
ness, reaches  sometimes  as  far  as  the  polar  star.  On  some  rare  occa- 
sions, one  or  two,  and  even  three,  concentric  arches  of  bright  light 
form  one  above  the  other  over  the  dark  segment,  where  they  appear 
as  brilliant  concentric  rainbows.  While  the  aurora  continues  to  de- 
velop and  spread  out  its  immense  arc,  the  border  of  the  dark  seg- 
ment loses  its  regularity  and  appears  indented  at  several  places  by 
patches  of  light,  which  soon  develop  into  long,  narrow,  diverging 
rays  and  streamers  of  great  beauty.  For  the  most  part  the  auroral 
light  is  either  whitish  or  of  a  pale,  greenish  tint;  but  in  some  cases  it 
exhibits  the  most  beautiful  colors,  among  which  the  red  and  green 
predominate.  In  these  cases  the  rays  and  streamers,  which  are 
usually  of  different  colors,  produce  the  most  magnificent  effects  by 
their  continual  changes  and  transformations. 


ASTRONOMICAL     DRAWINGS.  29 

The  brightness  and  extent  of  the  auroral  rays  are  likewise  sub- 
ject to  continual  changes.  An  instant  suffices  for  their  development 
and  disappearance,  which  may  be  succeeded  by  the  sudden  appear- 
ance of  others  elsewhere,  as  though  the  original  streamers  had  been 
swiftly  transported  to  a  new  place  while  invisible.  It  frequently 
happens  that  all  the  streamers  seem  to  move  sidewise,  from  west  to 
east,  along  the  arch,  continuing  meanwhile  to  exhibit  their  various 
changes  of  form  and  color.  For  a  time,  these  appearances  of  motion 
continue  to  increase,  a  succession  of  streamers  alternately  shooting 
forth  and  again  fading,  when  a  sudden  lull  occurs,  during  which  all  mo- 
tion seems  to  have  ceased.  The  stillness  then  prevailing  is  soon  suc- 
ceeded by  slight  pulsations  of  light,  which  seem  to  originate  on  the 
border  of  the  dark  segment,  and  are  propagated  upwards  along  the 
streamers,  which  have  now  become  more  numerous  and  active.  Slow 
at  first,  these  pulsations  quicken  by  degrees,  and  after  a  few  minutes 
the  whole  northern  sky  seems  to  be  in  rapid  vibration.  The  lively 
upward  and  downward  movement  of  these  streamers  entitles  them 
to  the  name  of  "  merry  dancers"  given  them  in  northern  countries 
where  they  are  frequent. 

Long  waves  of  light,  quickly  succeeded  by  others,  are  propa- 
gated in  an  instant  from  the  horizon  to  the  zenith;  these,  in  their 
rapid  passage,  cause  bends  and  curves  in  the  streamers,  which  then, 
losing  their  original  straightness,  wave  and  undulate  in  graceful 
folds,  resembling  those  of  a  pennant  in  a  gentle  breeze.  Although 
the  coruscations  add  to  the  grandeur  of  the  spectacle,  they  tend  to 
destroy  the  diverging  streamers,  which,  being  disconnected  from  the 
dark  segment,  or  torn  in  various  ways,  are,  as  it  were,  bodily  car- 
ried up  towards  the  zenith. 

In  this  new  phase  the  aurora  is  transformed  into  a  glorious 
crown  of  light,  called  the  "  Corona."  From  this  corona  diverge 
in  all  directions  long  streamers  of  different  colors  and  forms,  grace- 
fully undulating  in  numerous  folds,  like  so  many  banners  of  light. 
Some  of  the  largest  of  these  streamers  appear  like  fringes  composed 
of  short  transverse  rays  of  different  intensity  and  colors,  producing 
the  most  fantastic  effects,  when  traversed  by  the  pulsations  and 
coruscations  which  generally  run  across  these  rays  during  the  great 
auroral  displays. 

The  aurora  has  now  attained  its  full  development  and  beauty.  It 
may  continue  in  this  form  for  half  an  hour,  but  usually  the  celestial 
fires  begin  to  fade  at  the  end  of  fifteen  or  twenty  minutes,  reviving 
from  time  to  time,  but  gradually  dying  out.  The  northern  sky 
usually  appears  covered  by  gray  and  luminous  streaks  and  patches 


30  THE    TROUVELOT 

after  a  great  aurora,  these  being  occasionally  rekindled,  but  more 
often  they  gradually 'disappear,  and  the  sky  resumes  its  usual  ap- 
pearance. 

The  number  of  auroras  which  develop  a  corona  near  the  zenith 
is  comparatively  small  in  our  latitudes  ;  but  many  of  them,  although 
not  exhibited  on  so  grand  a  scale,  are  nevertheless  very  interesting. 
On  some  very  rare  occasions  the  auroral  display  has  been  confined 
almost  exclusively  to  the  dark  segment,  which  appeared  then  as  if 
pierced  along  its  border  by  many  square  openings,  like  windows, 
through  which  appeared  the  bright  auroral  light. 

Among  the  many  auroras  which  I  have  had  occasion  to  observe, 
none  are  more  interesting,  excepting  the  type  first  described,  than 
those  which  form  an  immense  arch  of  light  spanning  the  heavens 
from  East  to  West.  This  form  of  aurora,  which  is  quite  rare,  I  last 
observed  on  September  I2th,  1881.  All  the  northern  sky  was  cov- 
ered with  light  vapors,  when  a  small  auroral  patch  appeared  in  the 
East  at  about  20°  above  the  horizon.  This  patch  of  light,  gradually 
increasing  westward,  soon  reached  the  zenith,  and  continued  its 
onward  progress  until  it  arrived  at  about  20°  above  the  western 
horizon,  where  it  stopped.  The  aurora  then  appeared  as  a  narrow, 
wavy  band  of  light,  crossed  by  numerous  parallel  rays  of  different 
intensity  and  color.  These  rays  seemed  to  have  a  rapid  motion 
from  West  to  East  along  the  delicately-fringed  streamer,  which,  on 
the  whole,  moved  southward,  while  its  extremities  remained  undis- 
turbed. Aside  from  the  apparent  displacement  of  the  fringes,  a 
singular  vibrating  motion  was  observed  in  the  auroral  band,  which 
was  traversed  by  pulsations  and  long  waves  of  light.  The  phenom- 
ena lasted  for  about  twenty  minutes,  after  which  the  arch  was 
broken  in  many  places,  and  it  slowly  vanished. 

The  aurora  usually  appears  in  the  early  part  of  the  evening,  and 
attains  its  full  development  between  ten  and  eleven  o'clock.  Al- 
though the  auroral  light  may  have  apparently  ceased,  yet  the  phe- 
nomenon is  not  at  an  end,  as  very  often  a  solitary  ray  is  visible  from 
time  to  time  ;  and  even  towards  morning  these  rays  sometimes  be- 
come quite  numerous.  On  some  occasions  the  phenomenon  even 
continues  through  the  following  day,  and  is  manifested  by  the  radial 
direction  of  the  cirrus-clouds  in  the  heights  of  our  atmosphere.  In 
1872  I,  myself,  observed  an  aurora  which  apparently  continued  for 
two  or  three  consecutive  days  and  nights.  In  August,  1859,  the 
northern  lights  remained  visible  in  the  United  States  for  a  whole 
week. 

The  height  attained  by  these  meteors  is  considerable,  and  it  is 


ASTRONOMICAL     DRA  WINGS.  31 

now  admitted  that  they  are  produced  in  the  rarefied  air  of  the  upper 
regions  of  our  atmosphere.  From  the  researches  of  Professor  Elias 
Loomis  on  the  great  auroras  observed  in  August  and  September, 
1859,  it  was  ascertained  that  the  inferior  part  of  the  auroral  rays  had 
an  altitude  of  46  miles,  while  that  of  their  summits  was  428  miles. 
These  rays  had,  therefore,  a  length  of  382  miles.  From  the  obser- 
vation of  thirty  auroral  displays,  it  has  been  found  that  the  mean 
height  attained  by  the  summit  of  these  streamers  above  the  Earth's 
surface  was  450  miles. 

But  if  the  auroral  streamers  are  generally  manifested  at  great 
heights  in  our  atmosphere,  it  would  appear  from  the  observations  of 
persons  living  in  the  regions  where  the  auroras  are  most  frequent,  as 
also  from  those  who  have  been  stationed  in  high  northern  and 
southern  latitudes,  that  the  phenomenon  sometimes  descends  very 
low.  Both  Sabine  and  Parry  saw  the  auroral  rays  projected  on  a 
distant  mountain  ;  Ross  saw  them  almost  at  sea-level  projected  on 
the  polar  ice  ;  while  Wrangel,  Franklin,  and  others  observed  similar 
phenomena.  Dr.  Hjaltalin,  who  has  lived  in  latitude  64°  46'  north, 
and  has  made  a  particular  study  of  the  aurora,  on  one  occasion  saw 
the  aurora  much  below  the  summit  of  a  hill  1,600  feet  high,  which 
was  not  very  far  off. 

The  same  aurora  is  sometimes  observed  on  the  same  night  at 
places  very  far  distant  from  one  another.  The  great  aurora  borealis 
of  August  28th,  1859,  for  instance,  was  seen  over  a  space  occupying 
150°  in  longitude — from  California  to  the  Ural  Mountains  in  Russia. 
It  even  appears  now  very  probable  that  the  phenomenon  is  universal 
,  on  our  globe,  and  that  the  northern  lights  observed  in  our  hemi- 
sphere are  simultaneous  with  the  aurora  australis  of  the  southern 
hemisphere.  The  aurora  of  September  2d,  1859,  was  observed  all 
through  North  and  South  America,  the  Sandwich  Islands,  Austra- 
lia, and  Africa;  the  streamers  and  pulsations  of  light  of  the  north 
pole  responding  to  the  rays  and  coruscations  of  the  south  pole.  Of 
thirty-four  auroras  observed  at  Hobart  Town,  in  Tasmania,  twenty- 
nine  corresponded  with  aurora  borealis  observed  in  our  hemisphere. 

The  auroral  phenomena,  although  sometimes  visible  within  the 
tropics,  are,  however,  quite  rare  in  these  regions.  For  the  most 
part  they  are  confined  within  certain  zones  situated  in  high  latitudes 
north  and  south.  The  zone  where  they  are  most  frequent  in  our 
hemisphere  forms  an  ellipse,  which  has  the  north  pole  at  one  of  its 
foci  ;  while  the  other  is  situated  somewhere  in  North  America,  in 
the  vicinity  of  the  magnetic  pole.  The  central  line  of  the  zone  upon 
which  the  auroras  seem  to  be  most  frequent  passes  from  the  north- 


32  THE     TROUVELOT 

ern  coast  of  Alaska  through  Hudson's  Bay  and  Labrador  to  Iceland,, 
and  then  follows  the  northern  coast  of  Europe  and  Asia.  The  num- 
ber of  auroras  diminishes  as  the  observer  recedes  from  this  zone, 
and  it  is  only  in  exceptional  cases  that  they  are  seen  near  the 
equator.  Near  the  pole  the  phenomenon  is  less  frequent  than  it  is- 
in  the  region  described.  In  North  America  we  occupy  a  favorable 
position  for  the  observation  of  auroras,  as  we  are  nearer  the  magnetic 
poles  than  are  the  Europeans  and  Asiatics,  and  we  consequently 
have  a  greater  number  of  auroras  in  corresponding  latitudes. 

The  position  of  the  dark  auroral  segment  varies  with  the  place 
occupied  by  the  observer,  and  its  centre  always  corresponds  with 
the  magnetic  meridian.  In  our  Eastern  States  the  auroral  segment 
appears  a  little  to  the  west  of  the  north  point  ;  but  as  the  observer 
proceeds  westward  it  gradually  approaches  this  point,  and  is  due 
north  when  seen  from  the  vicinity  of  Lake  Winnipeg.  At  Point 
Barrow,  in  the  extreme  northwest  of  the  United  States,  the  aurora 
is  observed  in  the  east.  In  Melville  Islands,  Parry  saw  it  in  the 
south  ;  while  in  Greenland  it  is  directly  in  the  west. 

It  is  stated  that  auroras  are  more  numerous  about  the  equinoxes 
than  they  are  at  any  other  seasons  ;  and  also,  when  the  earth  is  in 
perigee,  than  when  it  is  in  apogee.  An  examination  which  I  have 
made  of  a  catalogue  by  Professor  Loomis,  comprising  4,137  auroras 
observed  in  the  temperate  zone  of  our  hemisphere  from  1776  to  1873, 
sustains  this  statement.  During  this  period,  one  hundred  more 
auroras  were  recorded  during  each  of  the  months  comprising  the 
equinoxes,  than  during  any  other  months  of  the  year  ;  while  eighty 
more  auroras  were  observed  when  the  earth  was  in  perigee,  than 
when  it  was  in  apogee.  But  to  establish  the  truth  of  this  assertion 
on  a  solid  basis,  more  observations  in  both  hemispheres  will  be  re- 
quired. 

The  aurora  is  not  simply  a  terrestrial  phenomenon,  but  is  asso- 
ciated in  some  mysterious  way  with  the  conditions  of  the  Sun's- 
surface.  It  is  a  well-known  fact  that  terrestrial  magnetism  is 
influenced  directly  by  the  Sun,  which  creates  the  diurnal  oscillations 
of  the  magnetic  needle.  Between  sunrise  and  two  o'clock,  the  north 
pole  of  the  needle  moves  towards  the  west  in  our  northern  hemi- 
sphere, and  in  the  afternoon  and  evening  it  moves  the  other 
way.  These  daily  oscillations  of  the  needle  are  not  uniform  in 
extent  ;  they  have  a  period  of  regular  increase  and  decrease. 
At  a  given  place  the  daily  oscillations  of  the  magnetic  needle 
increase  and  decrease  with  regularity  during  a  period  which  is  equal 
to  10^  years.  As  this  period  closely  coincides  with  the  Sun- 


ASTRONOMICAL     DRAWINGS.  83 

spot  period,  the  connection  between  the  variation  of  the  needle  and 
these  solar  disturbances  has  been  recognized. 

Auroral  phenomena  generally  accompany  the  extraordinary 
perturbations  in  the  oscillations  of  the  magnetic  needle,  which  are 
commonly  called  "  magnetic  storms,"  and  the  greater  the  auroral 
displays,  the  greater  are  the  magnetic  perturbations.  Not  only  is 
the  needle  subject  to  unusual  displacements  during  an  aurora,  but 
its  movements  seem  to  be  simultaneous  ^with  the  pulsations  and 
waving  motions  of  the  delicate  auroral  streamers  in  the  sky.  When 
the  aurora  sends  forth  a  coruscation,  or  a  streamer  in  the  sky,  the 
magnetic  needle  responds  to  it  by  a  vibration.  The  inference  that 
the  auroral  phenomena  are  connected  with  terrestrial  magnetism  is 
further  supported  by  the  fact  that  the  centre  of  the  corona  is  always 
situated  exactly  in  the  direction  of  that  point  in  the  heavens  to 
which  the  dipping  needle  is  directed. 

It  has  been  found  that  the  aurora  is  a  periodical  phenomenon, 
and  that  its  period  corresponds  very  closely  with  those  of  the 
magnetic  needle  and  Sun-spots.  The  years  which  have  the  most 
Sun-spots  and  magnetic  disturbances  have  also  the  most  auroras. 
There  is  an  almost  perfect  similarity  between  the  courses  of  the 
three  sets  of  phenomena,  from  which  it  is  concluded  that  the  aurora 
is  connected  in  some  mysterious  way  with  the  action  of  the  Sun,  as 
well  as  with  the  magnetic  condition  of  the  earth. 

A  very  curious  observation,  which  has  been  supposed  to  have 
some  connection  with  this  subject,  was  made  on  Sept.  1st,  1859, 
by  Mr.  Carrington  and  Mr.  Hodgson,  in  England.  While  these 
observers,  who  were  situated  many  miles  from  one  another,  were 
both  engaged  at  the  same  time  in  observing  the  same  Sun-spot,  they 
suddenly  saw  two  luminous  spots  of  dazzling  brilliancy  bursting  into 
sight  from  the  edge  of  the  Sun-spot.  These  objects  moved  eastward 
for  about  five  minutes,  after  which  they  disappeared,  having  then 
traveled  nearly  34,000  miles.  Simultaneously  with  these  appear- 
ances, a  magnetic  disturbance  was  registered  at  Kew  by  the  self- 
registering  magnetic  instruments.  The  very  night  that  followed 
these  observations,  great  magnetic  perturbations,  accompanied  by 
brilliant  auroral  displays,  were  observed  in  Europe.  A  connection 
between  the  terrestrial  magnetism  and  the  auroral  phenomena  is 
further  proved  by  the  fact  that,  before  the  appearance  of  an  aurora, 
the  magnetic  intensity  of  our  globe  considerably  increases,  but 
diminishes  as  soon  as  the  first  flashes  show  themselves. 

The  auroral  phenomena  are  also  connected  in  some  way  with 
electricity,  and  generate  serious  disturbances  in  the  electric  currents 


34  THE     TROUVELOT 

traversing  our  telegraphic  lines,  which  are  thus  often  rendered 
useless  for  the  transmission  of  messages  during  great  auroral' 
displays.  It  sometimes  happens,  however,  during  such  displays,  that 
the  telegraphic  lines  can  be  operated  for  a  long  distance,  without  the 
assistance  of  a  battery  ;  the  aurora,  or  at  least  its  cause,  furnishing 
the  necessary  electric  current  for  the  working  of  the  line.  During 
auroras,  the  telephonic  lines  are  also  greatly  affected,  and  all  kinds 
of  noises  and  crepitations  are  heard  in  the  instruments. 

Two  observations  of  mine,  which  may  have  a  bearing  on  the  sub- 
ject, present  some  interest,  as  they  seem  to  indicate  the  action  of 
the  aurora  on  some  of  the  clouds  of  our  atmosphere.  On  January 
6th,  1872,  after  I  had  been  observing  a  brilliant  aurora  for  over 
one  hour,  an  isolated  black  cumulus  cloud  appeared  at  a  little  dis- 
tance from  the  western  extremity  of  the  dark  auroral  segment. 
This  cloud,  probably  driven  by  the  wind,  rapidly  advanced  east- 
ward, and  was  soon  followed  by  a  succession  of  similar  clouds,  all 
starting  from  the  same  point.  All  these  black  clouds  apparently 
followed  the  same  path,  which  was  not  a  straight  line,  but  paral- 
lel to  and  concentric  with  the  border  of  the  dark  auroral  seg- 
ment. When  the  first  cloud  arrived  in  the  vicinity  of  the  mag- 
netic meridian  passing  through  the  middle  of  the  auroral  arc,  it 
very  rapidly  dissolved,  and  on  reaching  this  meridian  became 
invisible.  The  same  phenomenon  was  observed  with  the  succession 
of  black  clouds  following,  each  rapidly  dissolving  as  it  approached 
the  magnetic  meridian.  This  phenomenon  of  black  clouds  vanishing 
like  phantoms  in  crossing  the  magnetic  meridian,  was  observed 
for  nearly  an  hour.  On  June  i/th,  1879,  I  observed  a  similar 
phenomenon  during  a  fine  auroral  display.  About  midway  between 
the  horizon  and  the  polar  star,  but  a  little  to  the  west  of  the 
magnetic  meridian,  there  was  a  large  black  cumulo-stratus  cloud 
which  very  slowly  advanced  eastward.  As  it  progressed  in  that 
direction,  its  eastern  extremity  was  dissolved  in  traversing  the 
magnetic  meridian;  while,  at  the  same  time,  several  short  and  quite 
bright  auroral  rays  issued  from  its  western  extremity,  which  in  its 
turn  dissolved  rapidly,  as  if  burned  or  melted  away  in  the  production 
of  the  auroral  flame. 

It  seems  to  be  a  well  observed  fact,  that  during  auroras,  a  strong 
sulphurous  odor  prevails  in  high  northern  latitudes.  According  to 
Dr.  Hjaltalin,  during  these  phenomena,  "the  ozone  of  the  atmosphere 
increases  considerably,  and  men  and  animals  exposed  out  of  doors 
emit  a  sulphurous  odor  when  entering  a  heated  room."  The 
Esquimaux  and  other  inhabitants  of  the  northern  regions  assert 


ASTRONOMICAL     DRAWINGS.  35 

that  great  auroras  are  sometimes  accompanied  by  crepitations  and 
crackling  noises  of  various  sorts.  Although  these  assertions  have 
been  denied  by  several  travelers  who  have  visited  the  regions  of  these 
phenomena,  they  are  confirmed  by  many  competent  observers.  Dr, 
Hjaltalin,  who  has  heard  these  noises  about  six  times  in  a  hundred 
observations,  says  that  they  are  especially  audible  when  the  weather 
is  clear  and  calm  ;  but  that  when  the  atmosphere  is  agitated  they 
are  not  heard.  He  compares  them  to  the  peculiar  sound  produced 
by  a  silk  cloth  when  torn  asunder,  or  to  the  crepitations  of  the 
electric  machine  when  its  motion  is  accelerated.  "  When  the  auroral 
light  is  much  agitated  and  the  streamers  show  great  movements, 
it  is  then  that  these  noises  are  heard  at  different  places  in  the 
atmosphere." 

The  spectrum  of  the  auroral  light,  although  it  varies  with  al- 
most every  aurora,  always  shows  a  bright  green  line  on  a  faint 
continuous  spectrum.  In  addition  to  this  green  line  I  have  fre- 
quently observed  four  broad  diffused  bands  of  greater  refrangibility 
in  the  spectra  of  some  auroras.  In  two  cases,  when  the  auroras 
appeared  red  towards  the  west,  the  spectrum  showed  a  bright  red 
line,  in  addition  to  the  green  line  and  the  broad  bands  described. 
These  facts  evidently  show  that  the  light  of  the  aurora  is  due  to 
the  presence  of  luminous  vapors  in  our  atmosphere  ;  and  it  may 
reasonably  be  supposed  that  these  vapors  are  rendered  luminous  by 
the  passage  of  electric  discharges  through  them. 


36  THE    TROUVELOT 


THE  ZODIACAL  LIGHT. 
PLATE  V. 

IN  our  northern  latitudes  may  be  seen,  on  every  clear  winter  and 
spring  evening,  a  column  of  faint,  whitish,  nebulous  light,  rising  ob- 
liquely above  the  western  horizon.  A  similar  phenomenon  may  also 
be  observed  in  the  east,  before  day-break,  on  any  clear  summer  or 
autumn  night.  To  this  pale,  glimmering  luminosity  the  name  of 
"  Zodiacal  Light"  has  been  given,  from  the  fact  that  it  lies  in  the 
zodiac  along  the  ecliptic. 

In  common  with  all  the  celestial  bodies,  the  zodiacal  light  parti- 
cipates in  the  diurnal  motion  of  the  sky,  and  rises  and  sets  with  the 
constellations  in  which  it  appears.  Aside  from  this  apparent  motion, 
it  is  endowed  with  a  motion  of  its  own,  accomplished  from  west  to 
east,  in  a  period  of  a  year.  In  its  motion  among  the  stars,  the 
zodiacal  light  always  keeps  pace  with  the  Sun,  and  appears  as  if 
forming  two  faint  luminous  wings,  resting  on  opposite  sides  of  this 
body.  In  reality  it  extends  on  each  side  of  the  Sun,  its  axis  lying 
very  nearly  in  the  plane  of  the  ecliptic. 

In  our  latitudes  the  phenomena  can  be  observed  most  advan- 
tageously towards  the  equinoxes,  in  March  and  September,  when 
twilight  is  of  short  duration.  As  we  proceed  southward  it  becomes 
more  prominent,  and  gradually  increases  in  size  and  brightness.  It 
is  within  the  tropical  regions  that  the  zodiacal  light  acquires  all  its 
splendor  :  there  it  is  visible  all  the  year  round,  and  always  appears 
very  nearly  perpendicular  to  the  horizon,  while  at  the  same  time  its 
proportions  and  brilliancy  are  greatly  increased. 

The  zodiacal  light  appears  under  the  form  of  a  spear-head,  or  of 
a  narrow  cone  of  light  whose  base  apparently  rests  on  the  horizon, 
while  its  summit  rises  among  the  zodiacal  constellations.  In  gen- 
eral appearance  it  somewhat  resembles  the  tail  of  a  large  comet 
whose  head  is  below  the  horizon.  The  most  favorable  time  to  ob- 
serve this  phenomenon  in  the  evening,  is  immediately  after  the  last 
trace  of  twilight  has  disappeared  ;  and  in  the  morning,  one  or  two 


ASTRONOMICAL     DRAWINGS.  37 

hours  before  twilight  appears.  When  observed  with  attention,  it  is 
seen  that  the  light  of  the  zodiacal  cone  is  not  uniform,  but  gradually 
increases  in  brightness  inwardly,  especially  towards  its  base,  where 
it  sometimes  surpasses  in  brilliancy  the  brightest  parts  of  the  Milky- 
Way.  In  general,  its  outlines  are  vague  and  very  difficult  to  make 
out,  so  gradually  do  they  blend  with  the  sky.  On  some  favorable 
occasions,  the  luminous  cone  appears  to  be  composed  of  several  dis- 
tinct concentric  conical  layers,  having  different  degrees  of  bright- 
ness, the  inner  cone  being  the  most  brilliant  of  all.  There  is  a  re- 
markable distinction  between  the  evening  and  morning  zodiacal 
light.  In  our  climate,  the  morning  light  is  pale,  and  never  so  bright 
nor  so  extended  as  the  evening  light. 

In  general,  the  zodiacal  light  is  whitish  and  colorless,  but  in 
some  cases  it  acquires  a  warm  yellowish  or  reddish    tint.     These 
changes  of  color  may  be  accidental  and  due  to  atmospheric  condi- 
tions, and  not  to  actual  change  in  the  color  of  the  object.     Although 
the  zodiacal  light  is  quite  bright,  and  produces  the  impression  of 
having  considerable  depth,  yet  its  transparency  is  great,  since  all 
the  stars,  except  the  faint  ones,  can  be  seen  through  its  substance. 
The  zodiacal  light  is  subject  to  considerable  variations  in  bright- 
ness, and  also  varies  in  extent,  the  apex  of  its  cone  varying  in  dis- 
tance from  the  Sun's  place,  from  40  to  90  degrees.    These  variations 
cannot  be  attributed  to  atmospheric  causes  alone,  some  of  them 
being  due  to  real  changes  in  the  zodiacal  light  itself,  whose  light 
and  dimensions  increase  or  decrease  under  the  action  of  causes  at 
present  unknown.     From  the  discussion  of  a  series  of  observations 
on  the  zodiacal  light  made  at  Paris  and  Geneva,  it  appears  certain 
that  its  light  varies  from  year  to  year,  and  sometimes  even  from  day 
to   day,  independently  of  atmospheric  causes.     Some  of  my  own 
observations  agree  with  these  results,  and  one  of  them,  at  least, 
seems  to  indicate  changes  even  more  rapid.     On  December  i8th, 
1875,   I  observed  the   zodiacal  light  in  a  clear  sky  free  from  any 
vapors,  at  six  o'clock  in  the  evening.     At  that  time,  the  point  of  its 
cone  was  a  little  to  the  north  of  the  ecliptic,  at  a  distance  of  about 
90  degrees  from  the  Sun's  place.     Ten  minutes  later,  its  summit  had 
sunk  down  35  degrees,  the  cone  then  being  reduced  to  nearly  one- 
half  of  its  original  dimensions.     Ten  minutes  later,  it  had  risen  25 
degrees,   and  was  then  80  degrees  from  the  Sun's  place,  where  it 
remained  all  the  evening.     On  March  22d,   1878,  the  sky  was  very 
clear  and  the  zodiacal  light  was  bright  when  I  observed  it,  at  eight 
o'clock.     At  that  moment  the  apex  of  the  cone  of  light  was  a  little 
to  the  south  of  the  Pleiades,  but  this  cone  presented  an  unusual 


SS8  THE    TROUVELOT 

appearance  never  noticed  by  me  before,  its  northern  border  appearing 
much  brighter  and  sharper  than  usual,  while  at  the  same  time  its 
axis  of  greatest  brightness  appeared  to  be  much  nearer  to  this 
northern  border  than  it  was  to  the  southern.  After  a  few  minutes 
of  observation  it  became  evident  that  the  northern  border  was 
extending  itself,  as  stars  which  were  at  some  distance  from  it 
became  gradually  involved  in  its  light.  At  the  same  time  that 
this  border  spread  northward,  it  seemed  to  diffuse  itself,  and  after  a 
time  the  cone  presented  its  usual  appearance,  having  its  southern 
border  brighter  and  better  defined  than  the  other.  It  would  have 
been  impossible  to  attribute  this  sudden  change  to  an  atmospheric 
cause,  since  only  one  of  the  borders  of  the  cone  participated  in  it, 
and  since  some  very  faint  stars  near  this  northern  border  were  not 
affected  in  the  least  while  the  phenomenon  occurred.  Besides  these 
observations,  Cassini,  Mairan,  Humboldt,  and  many  other  competent 
observers  have  seen  pulsations,  coruscations  and  flickerings  in  the 
light  of  the  cone,  which  they  thought  could  not  be  attributed  to 
atmospheric  causes.  It  has  also  been  observed  that  at  certain 
periods  the  zodiacal  light  has  shone  with  unusual  intensity  for 
months  together. 

When  this  phenomenon  is  observed  from  the  tropical  regions,  it 
is  found  that  its  axis  of  symmetry  always  corresponds  with  its  axis 
of  greatest  brightness,  and  that  both  lie  in  the  plane  of  the  ecliptic, 
which  divides  its  cone  into  two  equal  parts.  But  when  the  zodiacal 
light  is  observed  in  our  latitude,  the  axis  of  symmetry  does  not 
correspond  with  the  axis  of  greatest  brightness,  and  both  axes  are 
a  little  to  the  north  of  this  plane,  the  axis  of  symmetry  being  the 
farther  removed.  Furthermore,  as  already  stated,  the  southern  bor- 
der of  the  cone  always  appears  better  defined  and  brighter  than  the 
corresponding  northern  margin.  It  is  very  probable,  if  not  abso- 
lutely certain,  that  these  phenomena  are  exactly  reversed  when 
the  zodiacal  light  is  observed  from  corresponding  latitudes  in  the 
southern  hemisphere,  and  that  there,  its  axes,  both  of  symmetry  and 
of  greatest  brightness,  appear  south  of  the  ecliptic,  while  the  northern 
margin  is  the  brightest.  This  seems  to  be  established  by  the  valu- 
able observations  of  Rev.  George  Jones,  made  on  board  the  U.  S. 
steam  frigate  Mississippi,  in  California,  Japan,  and  the  Southern 
Ocean.  "When  I  was  north  of  the  ecliptic,"  says  this  observer, 
"  the  greatest  part  of  the  light  of  the  cone  appeared  to  the  north  of 
this  line  ;  when  I  was  to  the  south  of  the  ecliptic,  it  appeared  to  be 
south  of  it  ;  while  when  my  position  was  on  the  ecliptic,  or  in  its 
vicinity,  the  zodiacal  cone  was  equally  divided  by  this  line." 


ASTRONOMICAL     DRA  WINGS.  39 

Besides  the  zodiacal  light  observed  in  the  East  and  West,  some 
observers  have  recognized  an  exceedingly  faint,  luminous,  gauzy 
band,  about  10  or  12  degrees  wide,  stretching  along  the  ecliptic  from 
the  summit  of  the  western  to  that  of  the  eastern  zodiacal  cone.  This 
faint  narrow  belt  has  been  called  the  Zodiacal  Band.  It  has  been 
recognized  by  Mr.  H.  C.  Lewis,  who  has  made  a  study  of  this 
phenomenon,  that  the  zodiacal  band  has  its  southern  margin  a  little 
brighter  and  a  little  sharper  than  the  northern  border.  This 
observation  is  in  accordance  with  similar  phenomena  observed  in 
the  zodiacal  light,  and  may  have  considerable  importance. 

In  1854,  Brorsen  recognized  a  faint,  roundish,  luminous  spot  in  a 
point  of  the  heavens  exactly  opposite  to  the  place  occupied  by 
the  Sun,  which  he  has  called  "  Gegenschein,"  or  counter-glow. 
This  luminous  spot  has  sometimes  a  small  nucleus,  which  is  a  little 
brighter  than  the  rest.  Night  after  night  this  very  faint  object  shifts 
its  position  among  the  constellations,  keeping  always  at  1 80  degrees 
from  the  Sun.  The  position  of  the  counter-glow,  like  that  of  the 
zodiacal  light  and  zodiacal  band,  is  not  precisely  on  the  plane  of  the 
ecliptic,  but  a  little  to  the  north  of  this  line.  It  is  very  probable 
that  near  the  equator  the  phenomenon  would  appear  different  and 
there  would  correspond  with  this  plane. 

There  seems  to  be  some  confusion  among  observers  in  regard  to 
the  spectrum  of  the  zodiacal  light.  Some  have  seen  a  bright  green 
line  in  its  spectrum,  corresponding  to  that  of  the  aurora  borealis  ; 
while  others  could  only  see  a  faint  grayish  continuous  spectrum, 
which  differs,  however,  from  that  of  a  faint  solar  light,  by  the  fact  that 
it  presents  a  well-defined  bright  zone,  gradually  blending  on  each  side 
with  the  fainter  light  of  the  continuous  spectrum.  I  have,  myself, 
frequently  observed  the  faint  continuous  spectrum  of  the  zodiacal 
light,  and  on  one  occasion  recognized  the  green  line  of  the  aurora  ; 
but  it  might  have  been  produced  by  the  aurora  itself,  as  yet  invisi- 
ble to  the  eye,  and  not  by  the  zodiacal  light,  since,  later  in  the  same 
evening,  there  was  a  brilliant  auroral  display.  If  it  were  demon- 
strated that  this  green  line  exists  in  the  spectrum  of  the  zodiacal 
light,  the  fact  would  have  importance,  as  tending  to  show  that  the 
aurora  and  the  zodiacal  light  have  a  common  origin. 

Rev.  Geo.  Jones  describes  a  very  curious  phenomenon  which  he 
observed  several  times  a  little  before  the  moon  rose  above  the  hori- 
zon. The  phenomenon  consisted  in  a  short,  oblique,  luminous  cone 
rising  from  the  Moon's  place  in  the  direction  of  the  ecliptic.  This 
phenomenon  he  has  called  the  Moon  Zodiacal  Light.  In  1874,  I  had 
an  opportunity  to  observe  a  similar  phenomenon  when  the  Moon 


40  THE    TROUVELOT 

was  quite  high  in  the  sky.  By  taking  the  precaution  to  screen  the 
Moon's  disk  by  the  interposition  of  some  buildings  between  it  and 
my  eye,  I  saw  two  long  and  narrow  cones  of  light  parallel  to  the 
ecliptic  issuing  from  opposite  sides  of  our  satellite.  The  phe- 
nomenon could  not  possibly  be  attributed  to  vapors  in  our  atmo- 
sphere, since  the  sky  was  very  clear  at  the  moment  of  the  observa- 
tion. Later  on,  these  appendages  disappeared  with  the  formation 
of  vapors  near  the  Moon,  but  they  reappeared  an  hour  later, 
when  the  sky  had  cleared  off,  and  continued  visible  for  twenty  min- 
utes longer,  and  then  disappeared  in  a  clear  sky. 

Although  the  zodiacal  light  has  been  studied  for  over  two  cen- 
turies, no  wholly  satisfactory  explanation  of  the  phenomenon  has 
yet  been  given.  Now,  as  in  Cassini's  time,  it  is  generally  consid- 
ered by  astronomers  to  be  due  to  a  kind  of  lens-shaped  ring  sur- 
rounding the  Sun,  and  extending  a  little  beyond  the  Earth's  orbit. 
This  ring  is  supposed  to  lie  in  the  plane  of  the  ecliptic,  and  to  be  com- 
posed of  a  multitude  of  independent  meteoric  particles  circulating  in 
closed  parallel  orbits  around  the  Sun.  But  many  difficulties  lie  in 
the  way  of  this  theory.  It  seems  as  incompetent  to  explain  the 
slow  and  rapid  changes  in  the  light  of  this  object  as  it  is  to  explain 
the  contractions  and  extensions  of  its  cone.  It  fails,  moreover,  to 
explain  the  flickering  motions,  the  coruscations  observed  in  its 
light,  or  the  displacement  of  its  cone  and  of  its  axes  of  brightness 
and  symmetry  by  a  mere  change  in  the  position  of  the  observer. 
Rev.  Geo.  Jones,  unable  to  explain  by  this  theory  the  phenomena 
which  came  under  his  observation,  has  proposed  another,  which  sup- 
poses the  zodiacal  light  to  be  produced  by  a  luminous  ring  surround- 
ing the  Earth,  this  ring  not  extending  as  far  as  the  orbit  of  the 
Moon.  But  this  theory  also  fails  in  many  important  points,  so  that 
at  present  no  satisfactory  explanation  of  the  phenomenon  can  be 
given. 

As  the  phenomenon  is  connected  in  some  way  with  the  Sun,  and 
as  we  have  many  reasons  to  believe  this  body  to  be  always  more  or 
less  electrified,  it  might  be  supposed  that  the  Sun,  acting  by  induc- 
tion on  our  globe,  develops  feeble  electric  currents  in  the  rarefied 
gases  of  the  superior  regions  of  our  atmosphere,  and  there  forms  a 
kind  of  luminous  ridge  moving  with  the  Sun  in  a  direction  contrary 
to  the  diurnal  motion,  and  so  producing  the  zodiacal  light.  On  this 
hypothesis,  the  counter-glow  would  be  the  result  of  a  smaller  cone 
of  light  generated  by  the  solar  induction  on  the  opposite  point  of 
the  Earth. 

Plate  5,  which  sufficiently  explains  itself,  represents  the  zodiacal 


ASTRONOMICAL     DRAWINGS.  41 

light  as  it  appeared  in  the  West  on  the  evening  of  February  2Oth, 
1876.  All  the  stars  are  placed  in  their  proper  position,  and  their 
relative  brightness  is  approximately  shown  by  corresponding  varia- 
tions in  size — the  usual  and  almost  the  only  available  means  of  repre- 
sentation. Of  course,  it  must  be  remembered  that  a  star  does  not, 
in  fact,  show  any  disk  even  in  the  largest  telescopes,  where  it  ap- 
pears as  a  mere  point  of  light,  having  more  or  less  brilliancy.  The 
cone  of  light  rises  obliquely  along  the  ecliptic,  and  the  point  forming 
its  summit  is  found  in  the  vicinity  of  the  well-known  group  of  stars, 
called  the  Pleiades,  in  the  constellation  of  Taurus,  or  the  Bull. 


42  THE    TROUVELOT 


THE  MOON. 

PLATE  VI. 

IN  its  endless  journey  through  space,  our  globe  is  not  solitary, 
like  some  of  the  planets,  but  is  attended  by  the  Moon,  our  nearest 
celestial  neighbor.  Although  the  Moon  does  not  attain  to  the 
dignity  of  a  planet,  and  remains  a  secondary  body  in  the  solar 
system,  yet,  owing  to  its  proximity  to  our  globe,  and  to  the  great 
influence  it  exerts  upon  it  by  its  powerful  attraction,  it  is  to  us  one 
of  the  most  important  celestial  bodies. 

While  the  Moon  accompanies  the  Earth  around  the  Sun,  it  also 
revolves  around  the  Earth  at  a  mean  distance  of  238,800  miles.  For 
a  celestial  distance  this  is  only  a  trifling  one  ;  the  Earth  in  advanc- 
ing on  its  orbit  travels  over  such  a  distance  in  less  than  four  hours. 
A  cannon  ball  would  reach  our  satellite  in  nine  days  ;  and  a  tele- 
graphic dispatch  would  be  transmitted  there  in  iJ/2  seconds  of  time, 
if  a  wire  could  be  stretched  between  us  and  the  Moon. 

Owing  to  the  ellipticity  of  the  Moon's  orbit,  its  distance  from 
the  Earth  varies  considerably,  our  satellite  being  sometimes  38,000 
miles  nearer  to  us  than  it  is  at  other  times.  These  changes  in  the 
distance  of  the  Moon  occasion  corresponding  changes  from  29'  to  33' 
in  its  apparent  diameter.  The  real  diameter  of  the  Moon  is  2,160 
miles,  or  a  little  over  one-quarter  the  diameter  of  our  globe  ;  our 
satellite  being  49  times  smaller  than  the  Earth. 

The  mean  density  of  the  materials  composing  the  Moon  is  only 
T%  that  of  the  materials  composing  the  Earth,  and  the  force  of  gravi- 
tation at  the  surface  of  our  satellite  is  six  times  less  than  it  is  at  the 
surface  of  our  globe.  If  a  person  weighing  150  Ibs.  on  our  Earth 
could  be  transported  to  the  Moon,  his  weight  there  would  be  only 
25  Ibs. 

The  Moon  revolves  around  the  Earth  in  about  27*^  days,  with  a 
mean  velocity  of  one  mile  per  second,  the  revolution  constituting  its 
sidereal  period.  If  the  Earth  were  motionless,  the  lunar  month  would 
be  equal  to  the  sidereal  period  ;  but  owing  to  its  motion  in  space, 


ASTRONOMICAL     DRA  WINGS.  43 

the  Sun  appears  to  move  with  the  Moon,  though  more  slowly,  so  that 
after  having  accomplished  one  complete  revolution,  our  satellite  has 
yet  to  advance  2^  days  before  reaching  the  same  apparent  position 
in  regard  to  the  Earth  and  the  Sun  that  it  had  at  first.  The  interval 
of  time  comprised  between  two  successive  New  Moons,  which  is  a 
little  over  29^  days,  constitutes  the  synodical  period  of  the  Moon, 
or  the  lunar  month. 

The  Moon  is  not  a  self-luminous  body,  but,  like  the  Earth  and  the 
planets,  it  reflects  the  light  which  it  receives  from  the  Sun,  and  so  ap- 
pears luminous.  That  such  is  the  case  is  sufficiently  demonstrated  by 
the  phases  exhibited  by  our  satellite  in  the  course  of  the  lunar  month. 
Every  one  is  familiar  with  these  phases,  which  are  a  consequence  of 
the  motion  of  the  Moon  around  the  Earth.  When  our  satellite 
is  situated  between  us  and  the  Sun,  it  is  New  Moon;  since  we  can- 
not see  its  illuminated  side,  which  is  then  turned  away  from  us 
towards  the  Sun.  When,  on  the  contrary,  it  reaches  that  point  of 
its  orbit  which,  in  regard  to  us,  is  opposite  to  the  Sun's  place,  it  is 
Full  Moon  ;  since  from  the  Earth  we  can  only  see  the  fully  illumin- 
ated side  of  our  satellite.  Again,  when  the  Moon  arrives  at  either 
of  the  two  opposite  points  of  its  orbit,  the  direction  of  which  from 
the  Earth  is  at  right  angles  with  that  of  the  Sun,  it  is  either  the 
First  or  the  Last  Quarter  ;  since  in  these  positions  we  can  only  see 
one-half  of  its  illuminated  disk. 

The  curve  described  by  the  Moon  around  the  Earth  lies  ap- 
proximately in  a  plane,  this  plane  being  inclined  about  5°  to  the 
ecliptic.  Since  our  satellite,  in  its  motion  around  us  and  the  Sun, 
closely  follows  the  ecliptic,  which  is  inclined  23^°  to  the  equator,  it 
results  that  when  this  plane  is  respectively  high  or  low  in  the  sky, 
the  moon  is  also  high  or  low  when  crossing  the  meridian  of  the  ob- 
server. In  winter  that  part  of  the  ecliptic  occupied  by  the  Sun  is 
below  the  equator,  and,  consequently,  the  New  Moons  occurring  in 
that  season  are  low  in  the  sky,  since  at  New  Moon  our  satellite 
must  be  on  the  same  side  of  the  ecliptic  with  the  Sun.  But  the  Full 
Moons  in  the  same  season  are  necessarily  high  in  the  sky,  since  a 
Full  Moon  can  only  occur  when  our  satellite  is  on  the  opposite  side 
of  the  ecliptic  from  the  Sun,  in  which  position  it  is,  of  course,  as 
many  degrees  above  the  equator  as  the  Sun  is  below.  The  Full 
Moon  which  happens  nearest  to  the  autumnal  equinox  is  commonly 
called  the  Harvest  Moon,  from  the  fact  that,  after  full,  its  delays  in 
rising  on  successive  evenings  are  very  brief  and  therefore  favorable 
for  the  harvest  work  in  the  evening.  The  same  phenomenon  occurs 
in  every  other  lunar  month,  but  not  sufficiently  near  the  time  of 


44  THE    TROUVELOT 

Full  Moon  to  be  noticeable.  When,  in  spring,  a  day  or  two  after 
New  Moon,  our  satellite  begins  to  show  its  thin  crescent,  its  position 
on  the  ecliptic  is  north  as  well  as  east  of  that  occupied  by  the  Sun  ; 
hence,  its  horns  are  nearly  upright  in  direction,  and  give  it  a  crude 
resemblance  to  a  tipping  bowl,  from  which  many  people  who  are 
unaware  of  its  cause,  and  that  this  happens  every  year,  draw  con- 
clusions as  to  the  amount  of  rain  to  be  expected. 

One  of  the  most  remarkable  features  of  the  Moon's  motions  is 
that  our  satellite  rotates  on  its  axis  in  exactly  the  same  period  of 
time  occupied  by  its  revolution  around  the  Earth,  from  which  it 
results  that  the  Moon  always  presents  to  us  the  same  face.  To 
explain  this  peculiarity,  astronomers  have  supposed  that  the  figure 
of  our  satellite  is  not  perfectly  spherical,  but  elongated,  so  that  the 
attraction  of  the  Earth,  acting  more  powerfully  upon  its  nearest 
portions,  always  keeps  them  turned  toward  us,  as  if  the  Moon 
were  united  to  our  globe  by  a  string.  It  is  not  exactly  true,  how- 
ever, that  the  Moon  always  presents  its  same  side  to  us,  although  its 
period  of  rotation  exactly  equals  that  of  its  revolution  ;  since  in 
consequence  of  the  inclination  of  its  axis  of  rotation  to  its  orbit, 
combined  with  the  irregularities  of  its  orbital  motion  about  us, 
apparent  oscillations  in  latitude  and  in  longitude,  called  librations, 
are  created,  from  which  it  results  that  nearly  -fa  of  the  Moon's  sur- 
face is  visible  from  the  Earth  at  one  time  or  another. 

The  Moon  is  a  familiar  object,  and  every  one  is  aware  that  our 
satellite,  especially  when  it  is  fully  illuminated,  presents  a  variety  of 
bright  and  dark  markings,  which,  from  their  distant  resemblance  to- 
a  human  face,  are  popularly  known  as  "  the  man  in  the  moon."  A 
day  or  two  after  New  Moon,  when  the  thin  crescent  of  our  satellite 
is  visible  above  the  western  horizon  after  sunset,  the  dark  portion 
of  its  disk  is  plainly  visible,  and  appears  of  a  pale,  ashy  gray  color, 
although  not  directly  illuminated  by  the  Sun.  This  phenomenon  is 
due  to  the  Earth-shine,  or  to  that  portion  of  solar  light  which  the 
illuminated  surface  of  our  globe  reflects  to  the  dark  side  of  the  Moon, 
exactly  in  the  same  manner  that  the  Moon-shine,  on  our  Earth,  is- 
due  to  the  solar  light  reflected  to  our  globe  by  the  illuminated  Moon. 

Seen  with  a  telescope  of  moderate  power,  or  even  with  a  good 
opera-glass,  the  Moon  presents  a  peculiar  mottled  appearance,  and 
has  a  strong  resemblance  to  a  globe  made  of  plaster  of  Paris,  on  the 
surface  of  which  numerous  roundish,  saucer-shaped  cavities  of  vari- 
ous sizes  are  scattered  at  random.  This  mottled  structure  is  better 
seen  along  the  boundary  line  called  the  terminator,  which  divides 
the  illuminated  from  the  dark  side  of  the  Moon.  The  line  of  the 


ASTRONOMICAL     DRAWINGS.  45 

terminator  always  appears  jagged,  and  it  is  very  easy  to  recognize 
that  this  irregularity  is  due  to  the  uneven  and  rugged  structure  of 
the  surface  of  our  satellite. 

A  glance  at  the  Moon  through  a  larger  telescope  shows  that  the 
bright  spots  recognized  with  the  naked  eye  belong  to  very  uneven 
and  mountainous  regions  of  our  satellite,  while  the  dark  ones  belong 
to  comparatively  smooth,  low  surfaces,  comparable  to  those  form- 
ing the  great  steppes  and  plains  of  the  Earth.  When  examined 
with  sufficient  magnifying  power,  the  white,  rugged  districts  of  the 
Moon  appear  covered  over  by  numerous  elevated  craggy  plateaus, 
mountain-chains,  and  deep  ravines  ;  by  steep  cliffs  and  ridges  ;  by 
peaks  of  great  height  and  cavities  of  great  depth.  This  rugged  form- 
ation, which  is  undoubtedly  of  volcanic  origin,  gives  our  satellite  a 
desolate  and  barren  appearance.  The  rugged  tract  occupies  more 
than  one-half  of  the  visible  surface  of  the  Moon,  forming  several  dis- 
tinct masses,  the  principal  of  which  occupy  the  south  and  south- 
western part  of  the  disk.  That  this  formation  is  elevated  above  the 
general  level  is  proved  by  the  fact  that  the  mountains,  peaks,  and 
other  objects  which  compose  it,  all  cast  a  shadow  opposite  to  the 
Sun  ;  and  further,  that  the  length  of  these  shadows  diminishes  with 
the  elevation  of  the  Sun  above  the  lunar  horizon. 

Since  Galileo's  time  the  surface  of  the  Moon  has  been  studied  by 
a  host  of  astronomers,  and  accurate  maps  of  its  topographical  con- 
figuration have  been  made,  and  names  given  to  all  features  of  any 
prominence.  It  may  even  be  said  that  in  its  general  features,  the 
visible  surface  of  our  satellite  is  now  better  known  to  us  than  is  the 
surface  of  our  own  Earth. 

One  of  the  most  striking  and  common  features  of  the  mountain- 
ous districts  of  the  Moon,  is  the  circular,  ring-like  disposition  of 
their  elevated  parts,  which  form  numerous  crater-like  objects  of  dif- 
ferent sizes  and  depths.  Many  thousands  of  crater-like  objects  are 
visible  on  the  Moon  through  a  good  telescope,  and,  considering  how 
numerous  the  small  ones  are,  there  is,  perhaps,  no  great  exaggera- 
tion in  fixing  their  number  at  50,000,  as  has  been  done  by  some 
astronomers.  These  volcanic  regions  of  the  Moon  cannot  be  com- 
pared to  anything  we  know,  and  far  surpass  in  extent  those  of  our 
globe.  The  number  and  size  of  the  craters  of  our  most  important 
volcanic  regions  in  Europe,  in  Asia,  in  North  and  South  America, 
in  Java,  in  Sumatra,  and  Borneo,  are  insignificant  when  com- 
pared with  those  of  the  Moon.  The  largest  known  craters  on 
the  Earth  give  only  a  faint  idea  of  the  magnitude  of  some  of 
the  lunar  craters.  The  great  crater  Haleakala,  in  the  Sandwich 


46  THE    TROUVELOT 

Islands,  probably  the  largest  of  the  terrestrial  volcanoes,  has  a  cir- 
cumference of  thirty  miles,  or  a  diameter  of  a  little  less  than  ten 
miles.  Some  of  the  great  lunar  craters,  called  walled  plains,  such 
as  Hipparchus,  Ptolemaeus,  etc.,  have  a  diameter  more  than  ten 
times  larger  than  that  of  Haleakala,  that  of  the  first  being  1 15  miles 
and  that  of  the  last  100  miles.  These  are,  of  course,  among  the 
largest  of  the  craters  of  the  Moon,  although  there  are  on  our  satel- 
lite a  great  number  of  craters  above  ten  miles  in  diameter. 

The  crater-forms  of  the  Moon  have  evidently  appeared  at  different 
periods  of  time,  since  small  craters  are  frequently  found  on  the  walls 
of  larger  ones  ;  and,  indeed,  still  smaller  craters  are  not  rarely  seen 
on  the  walls  of  these  last.  The  walls  of  the  lunar  craters  are  usu- 
ally quite  elevated  above  the  surrounding  surface,  some  of  them 
attaining  considerable  elevations,  especially  at  some  points,  which 
form  peaks  of  great  height.  Newton,  the  loftiest  of  all,  rises  at  one 
point  to  the  height  of  23,000  feet,  while  many  others  range  from 
ten  to  twenty  thousand  feet  in  height.  Several  craters  have  their 
floor  above  the  general  surface — Plato,  for  instance.  Wargentin 
has  its  floor  nearly  on  a  level  with  the  summit  of  its  walls, 
showing  that  at  some  period  of  its  history  liquid  lavas,  ejected 
from  within,  have  filled  it  to  the  brim  and  then  solidified.  The 
floors  of  some  of  the  craters  are  smooth  and  flat,  but  in  general 
they  are  occupied  by  peaks  and  abrupt  mountainous  masses,  which 
usually  form  the  centre.  Many  of  their  outside  walls  are  partly  or 
wholly  covered  by  numerous  ravines  and  gullies,  winding  down 
their  steep  declivities,  branching  out  and  sometimes  extending  to 
great  distances  from  their  base.  It  would  seem  that  these  great 
volcanic  mouths  have  at  some  time  poured  out  torrents  of  lavas, 
which,  in  their  descent,  carved  their  passage  by  the  deep  gullies 
now  visible.  Sometimes,  also,  the  crater  slopes  are  strewn  with 
debris,  giving  them  a  peculiar  volcanic  appearance. 

Notwithstanding  their  many  points  of  similarity  with  the  volca- 
noes of  the  Earth,  the  lunar  craters  differ  from  them  in  many  par- 
ticulars, showing  that  volcanic  forces  acting  on  different  globes  may 
produce  widely  different  results.  For  example,  the  floors  of  terres- 
trial craters  are  usually  situated  at  considerable  elevations  above 
the  general  surface,  while  those  of  the  lunar  craters  are  generally 
much  depressed,  the  height  of  their  walls  being  only  about  one- 
half  the  depth  of  their  cavities.  Again,  while  on  the  Earth  the 
mass  of  the  volcanic  cones  far  exceeds  the  capacity  of  their  open- 
ings, on  the  Moon  it  is  not  rare  to  see  the  capacity  of  the  crater 
cavities  exceeding  the  mass  of  the  surrounding  walls.  On  the 


ASTRONOMICAL     DRAWINGS.  47 

Earth,  the  volcanic  cones  and  mouths  are  comparatively  regular  and 
smooth,  and  are  generally  due  to  the  accumulation  of  the  ashes  and 
the  debris  of  all  kinds  which  are  ejected  from  the  volcanic  mouths. 
On  the  Moon,  very  few  craters  show  this  character,  and  for  the 
most  part  their  walls  have  a  very  different  structure,  being  irregu- 
lar, very  rugged,  and  composed  of  a  succession  of  conce'ntric  ridges, 
rising  at  many  points  to  great  elevations,  and  forming  peaks  of 
stupendous  height.  Again,  many  of  the  larger  terrestrial  craters 
have  their  interior  occupied  by  a  central  cone,  or  several  such  cones, 
having  a  volcanic  mouth  on  their  summits  ;  on  the  Moon  such  cen- 
tral cones  are  very  rare.  Although  many  of  the  large  lunar  craters 
have  their  interior  occupied  by  central  masses  which  have  been 
often  compared  to  the  central  cones  of  our  great  volcanoes,  yet 
these  objects  have  a  very  different  character  and  origin.  For  the 
most  part,  they  are  mountainous  masses  of  different  forms — having 
very  rarely  any  craters  on  them — and  seem  to  have  resulted  from 
the  crowding  and  lifting  up  of  the  crater  floor  by  the  phenomena 
of  subsidence,  of  which  these  craters  show  abundant  signs.  Besides, 
the  terrestrial  craters  are  characterized  by  large  and  important  lava 
streams,  while  on  the  Moon  the  traces  of  such  phenomena  are  quite 
rare,  and  when  they  are  shown,  they  generally  differ  from  those  of 
the  Earth  by  their  numerous  and  complicated  ramifications,  and 
also  by  the  fact  that  many  of  these  lava  streamlets  take  their  origin 
at  a  considerable  distance  from  the  crater  slopes,  and  are  grooved 
and  depressed  as  if  the  burning  liquids  which  are  supposed  to  have 
produced  them  had  subsequently  disappeared,  by  evaporation  or 
otherwise,  leaving  the  furrow  empty. 

The  dark  spots  of  the  Moon,  when  viewed  through  a  tele- 
scope, exhibit  a  totally  different  character,  and  show  that  they  be- 
long to  a  different  formation  from  that  of  the  brighter  portions. 
These  darker  tracts  do  not  seem  to  have  had  a  direct  volcanic  or- 
igin like  the  latter,  but  rather  appear  to  have  resulted  from  the  sol- 
idification of'semi-fluid  materials,  which  have  overflowed  vast  areas 
at  different  times.  The  surface  of  this  system  is  comparatively 
smooth  and  uniform,  only  some  small  craters  and  low  ridges  being 
seen  upon  it.  The  level  and  dark  appearance  of  these  areas 
led  the  ancient  astronomers  to  the  belief  that  they  were  produced 
by  a  liquid  strongly  absorbing  the  rays  of  light,  and  were  seas  like 
our  seas.  Accordingly,  these  dark  surfaces  were  called  Mariay  or 
Seas,  a  name  which  it  is  convenient  to  retain,  although  it  is  well 
known  to  have  originated  in  an  error.  The  so-called  seas  of  the 
Moon  are  evidently  large  flat  surfaces  similar  to  the  deserts,  steppes, 


48  THE    TROUVELOT 

pampas,  and  prairies  of  the  Earth  in  general  appearance.  The  great 
plains  of  the  Moon  are  at  a  lower  level  than  that  of  the  other  form- 
ation, and  that  which  first  attracts  the  observer's  attention  is  the 
fact  that  they  are  surrounded  almost  on  all  sides  by  an  irregular  line 
of  abrupt  cliffs  and  mountain  chains,  showing  phenomena  of  disloca- 
tion. This  character  of  dislocation,  which  is  general,  and  is  visible 
everywhere  upon  the  contours  of  the  plains,  seems  to  indicate  that 
phenomena  of  subsidence,  either  slow  or  rapid,  have  occurred  on  the 
Moon  ;  while,  at  the  same  time,  the  sunken  surfaces  were  overflowed 
by  a  semi-fluid  liquid,  which  solidified  afterwards.  The  evidences  of 
subsidence  and  overflowing  become  unmistakable  when  we  observe 
that,  along  the  borders  of  the  gray  plains,  numerous  craters  are  more 
or  less  embedded  in  the  gray  formation,  only  parts  of  the  summit  of 
their  walls  remaining  visible,  to  attest  that  once  large  craters  existed 
there.  The  farther  from  the  border  of  the  plain  the  vestiges  of  these 
craters  are  observed,  the  deeper  they  are  embedded  in  the  gray 
formation.  That  phenomena  of  subsidence  have  occurred  on  a  grand 
scale  on  the  Moon,  is  further  indicated  by  the  fact  that  the  singular 
systems  of  fractures  called  clefts  and  rifts  generally  follow  closely 
the  outside  border  of  the  gray  plains,  often  forming  parallel  lines  of 
dislocation  and  fractures.  In  the  interior  regions  of  the  gray  forma- 
tion, these  fractures  are  comparatively  rare. 

The  gray,  lava-like  formation  is  obviously  of  later  origin  than 
the  mountainous  system  to  which  belong  the  embedded  craters 
above  described.  Its  comparatively  recent  origin  might  also  be  in- 
ferred from  the  smallness  of  its  craters  and  its  low  ridges.  The  few 
large  craters  observed  on  this  formation  evidently  belong  to  the 
earlier  system. 

The  color  of  this  system  of  gray  plains  is  far  from  being  uniform. 
In  general  appearance  it  is  of  a  bluish  gray,  but  when  observed  at- 
tentively, large  areas  appear  tinted  with  a  dusky  olive-green,  while 
others  are  slightly  tinged  with  yellow.  Some  patches  appear  brown- 
ish, and  even  purplish.  A  remarkable  example  of  the  first  case  is 
seen  on  the  surface,  which  encloses  within  a  large  parallelogram  the 
two  conspicuous  craters,  Aristarchus  and  Herodotus.  This  surface 
evidently  belongs  to  a  different  system  from  that  of  the  Oceanus 
Procellarum  surrounding  it,  as,  besides  its  color,  which  totally  differs 
from  that  of  the  gray  formation,  its  surface  shows  the  rugged  struc- 
ture of  the  volcanic  formation. 

When  the  Moon  is  full,  some  very  curious  white,  luminous  streaks 
are  seen  radiating  from  different  centres,  which,  for  the  most  part,  are 
important  craters,  occupied  by  interior  mountains.  The  great  crater 


ASTRONOMICAL     DRA  WINGS.  49 

Tycho  is  the  centre  of  the  most  imposing  of  the  systems  of  white 
streaks.  Some  of  the  diverging  rays  of  this  great  centre  extend  to 
a  distance  equal  to  one-quarter  of  the  Moon's  circumference,  or  about 
i, 700  miles.  The  true  nature  of  these  luminous  streaks  is  unknown, 
but  it  seems  certain  that  they  have  their  origin  in  the  crater  from 
which  they  diverge.  They  do  not  form  any  relief  on  the  surface, 
and  are  seen  going  up  over  the  mountains  and  steep  walls  of  the 
crater,  as  well  as  down  the  ravines  and  on  the  floors  of  craters. 

The  Moon  seems  to  be  deprived  of  an  atmosphere ;  or,  if  it  has  any, 
it  must  be  so  excessively  rare  that  its  density  is  less  than  T^  of  the 
density  of  the  Earth's  atmosphere,  since  delicate  tests  afforded  by 
the  occultation  of  stars  have  failed  to  reveal  its  presence.  Although 
no  atmosphere  of  any  consequence  exists  on  the  Moon,  yet  phenom- 
ena which  I  have  observed  seem  to  indicate  the  occasional  presence 
there  of  vapors  of  some  sort.  On  several  occasions,  I  have  seen  a 
purplish  light  over  some  parts  of  the  Moon,  which  prevented  well- 
known  objects  being  as  distinctly  seen  as  they  were  at  other  times, 
causing  them  to  appear  as  if  seen  through  a  fog.  One  of  the  most 
striking  of  these  observations  was  made  on  January  4th,  1873,  on 
the  crater  Kant  and  its  vicinity,  which,  then  appeared  as  if  seen 
through  luminous  purplish  vapors.  On  one  occasion,  the  great  cra- 
ter Godin,  which  was  entirely  involved  in  the  shadow  of  its  western 
wall,  appeared  illuminated  in  its  interior  by  a  faint  purplish  light, 
which  enabled  me  to  recognize  the  structure  of  this  interior.  The 
phenomenon  could  not  be  attributed  in  this  case  to  reflection,  since 
the  Sun,  then  just  rising  on  the  western  wall  of  the  crater,  had  not 
yet  grazed  the  eastern  wall,  which  was  invisible.  It  is  not  impossi- 
ble that  a  very  rare  atmosphere  composed  of  such  vapors  exists  in 
the  lower  parts  of  the  Moon. 

If  the  Moon  has  no  air,  and  no  liquids  of  any  sort,  it  seems  im- 
possible that  its  surface  can  maintain  any  form  of  life,  either  vegeta- 
ble or  animal,  analogous  to  those  on  the  Earth.  In  fact,  nothing 
indicating  life  has  been  detected  on  the  Moon — our  satellite  look- 
ing like  a  barren,  lifeless  desert.  If  life  is  to  be  found  there  at  all,  it 
must  be  of  a  very  elementary  nature.  Aside  from  the  want  of  air 
and  water  to  sustain  it,  the  climatic  conditions  of  our  satellite  are 
very  unfavorable  for  the  development  of  life.  The  nights  and 
days  of  the  Moon  are  each  equal  to  nearly  fifteen  of  our  days  and 
nights.  For  fifteen  consecutive  terrestrial  days  the  Sun's  light  is 
absent  from  one  hemisphere  of  the  Moon  ;  while  for  the  same  num- 
ber of  days  the  Sun  pours  down  on  the  other  hemisphere  its  light 
and  heat,  the  effects  of  which  are  not  in  any  way  mitigated  by  an 


50  THE     TROUVELOT 

atmosphere.  During  the  long  lunar  nights  the  temperature  must  at 
least  fall  to  that  of  our  polar  regions,  while  during  its  long  days  it 
must  be  far  above  that  of  our  tropical  zone.  It  has  been  calculated 
that  during  the  lunar  nights  the  temperature  descends  to  23°  below 
zero,  while  during  the  days  it  rises  to  468°,  or  256°  above  the  boil- 
ing point. 

It  has  been  a  question  among  astronomers  whether  changes  are 
still  taking  place  at  the  surface  of  the  Moon.  Aside  from  the  fact 
that  change,  not  constancy,  is  the  law  of  nature,  it  does  not  seem 
doubtful  that  changes  occur  on  the  Moon,  especially  in  view  of 
the  powerful  influences  of  contraction  and  dilatation  to  which  its 
materials  are  submitted  by  its  severe  alternations  of  temperature. 
From  the  distance  at  which  we  view  our  satellite,  we  cannot  expect, 
of  course,  to  be  able  to  see  changes,  unless  they  are  produced  on  a 
large  scale.  Theoretically  speaking,  the  largest  telescopes  ever 
constructed  ought  to  show  us  the  Moon  as  it  would  appear  to  the 
naked  eye  from  a  distance  of  40  miles  ;  but  in  practice  it  is  very  dif- 
ferent. The  difficulty  is  in  the  fact  that,  while  we  magnify  the  sur- 
face of  a  telescopic  image,  we  are  unable  to  increase  its  light ;  so 
that,  practically,  in  magnifying  an  object,  we  weaken  its  light  pro- 
portionally to  the  magnifying  power  employed.  The  light  of  the 
Moon,  especially  near  the  terminator,  where  we  almost  always  make 
our  observations,  is  not  sufficiently  bright  to  bear  a  very  high  mag- 
nifying power,  and  only  moderate  ones  can  be  applied  to  its  study. 
What  we  gain  by  enlarging  an  object,  we  more  than  lose  by  the 
weakening  of  its  light.  Besides,  a  high  magnifying  power,  by 
increasing  the  disturbances  generally  present  in  our  atmosphere, 
renders  the  telescopic  image  unsteady  and  very  indistinct.  On  the 
whole,  the  largest  telescopes  now  in  existence  do  not  show  us  our 
satellite  better  than  if  we  could  see  it  with  the  naked  eye  from  a 
distance  of  300  miles  or  more.  At  such  a  distance  only  considerable 
changes  would  be  visible. 

Notwithstanding  these  difficulties,  it  is  believed  that  changes 
have  been  detected  in  Linne,  Marius,  Messier,  and  several  other  cra- 
ters. An  observation  of  mine  seems  to  indicate  that  changes  have 
recently  taken  place  in  the  great  crater  Eudoxus.  On  February 
2Oth,  1877,  between  9h.  3Om.  and  loh.  3Om.,  I  observed  a  straight, 
narrow  wall  crossing  this  crater  from  east  to  west,  a  little  to  the 
south  of  its  centre.  This  wall  had  a  considerable  elevation,  as  was 
proved  by  the  shadow  it  cast  on  its  northern  side.  Towards  its  west- 
ern end  this  wall  appeared  as  a  brilliant  thread  of  light  on  the  black 
shadow  cast  by  the  western  wall  of  the  crater.  The  first  time  I  had 


ASTRONOMICAL     DRAWINGS.  51 

occasion  to  observe  this  crater  again,  after  this  observation,  was  a 
year  later,  on  February  1 7th,  1878  ;  no  traces  of  the  wall  were  then 
detected.  Many  times  since  I  have  tried  to  find  this  narrow  wall 
again,  when  the  Moon  presented  the  same  phase  and  the  same 
illumination,  but  always  with  negative  results.  It  seems  probable 
that  this  structure  has  crumbled  down,  yet  it  is  very  singular  that 
so  prominent  a  feature  should  not  have  been  noticed  before. 

The  "  Mare  Humorum,"  or  sea  of  moisture,  as  it  is  called,  which 
is  represented  on  Plate  VI.,  is  one  of  the  smaller  gray  lunar  plains. 
Its  diameter,  which  is  very  nearly  the  same  in  all  directions,  is  about 
270  miles,  the  total  area  of  this  plain  being  about  50,000  square 
miles.  It  is  one  of  the  most  distinct  plains  of  the  Moon,  and  is 
easily  seen  with  the  naked  eye  on  the  left-hand  side  of  the  disk. 
The  floor  of  the  plain  is,  like  that  of  tjie  other  gray  plains,  traversed 
by  several  systems  of  very  extended  but  low  hills  and  ridges,  while 
small  craters  are  disseminated  upon  its  surface.  The  color  of  this 
formation  is  of  a  dusky  greenish  gray  along  the  border,  while  in  the 
interior  it  is  of  a  lighter  shade,  and  is  of  brownish  olivaceous  tint. 
This  plain,  which  is  surrounded  by  high  clefts  and  rifts,  well  illustrates 
the  phenomena  of  dislocation  and  subsidence.  The  double-ringed 
crater  Vitello,  whose  walls  rise  from  4,000  to  5,000  feet  in  height,  is 
seen  in  the  upper  left-hand  corner  of  the  gray  plain.  Close  to 
Vitello,  at  the  east,  is  the  large  broken  ring-plain  Lee,  and  farther 
east,  and  a  little  below,  is  a  similarly  broken  crater  called  Doppel- 
mayer.  Both  of  these  open  craters  have  mountainous  masses  and 
peaks  on  their  floor,  which  is  on  a  level  with  that  of  the  Mare  Hu- 
morum. A  little  below,  and  to  the  left  of  these  objects,  is  seen^a 
deeply  embedded  oval  crater,  whose  walls  barely  rise  above  the 
level  of  the  plain.  On  the  right-hand  side  of  the  great  plain,  is  a 
long  fault,  with  a  system  of  fracture  running  along  its  border.  On 
this  right-hand  side,  may  be  seen  a  part  of  the  line  of  the  terminator, 
which  separates  the  light  from  the  darkness.  Towards  the  lower  right- 
hand  corner,  is  the  great  ring-plain  Gassendi,  55  miles  in  diameter, 
with  its  system  of  fractures  and  its  central  mountains,  which  rise  from 
3,000  to  4,000  feet  above  its  floor.  This  crater  slopes  southward 
towards  the  plain,  showing  the  subsidence  to  which  it  has  been 
submitted.  While  the  northern  portion  of  the  wall  of  this  crater 
rises  to  10,000  feet,  that  on  the  plain  is  only  500  feet  high,  and  is 
even  wholly  demolished  at  one  place  where  the  floor  of  the  crater 
is  in  direct  communication  with  the  plain.  In  the  lower  part  of  the 
mare,  and  a  little  to  the  west  of  the  middle  line,  is  found  the  crater 
Agatharchides,  which  shows  below  its  north  wall  the  marks  of  rills 


52  THE    TROUVELOT 

impressed  by  a  flood  of  lava,  which  once  issued  from  the  side  of 
the  crater.  On  the  left-hand  side  of  the  plain,  is  seen  the  half- 
demolished  crater  Hippalus,  resembling  a  large  bay,  which  has  its 
interior  strewn  with  peaks  and  mountains.  On  this  same  side  can 
be  seen  one  of  the  most  important  systems  of  clefts  and  fractures 
visible  on  the  Moon,  these  clefts  varying  in  length  from  150  to  200 
miles. 


ASTRONOMICAL    DRAWINGS.  53 


ECLIPSES  OF  THE  MOON. 
PLATE  VII. 

SINCE  the  Moon  is  not  a  self-luminous  body,  but  shines  by  the  light 
which  it  borrows  from  the  Sun,  it  follows  that  when  the  Sun's  light  is 
prevented  from  reaching  its  surface,  our  satellite  becomes  obscured. 
The  Earth,  like  all  opaque  bodies  exposed  to  sunlight,  casts  a  sha- 
dow in  space,  the  direction  of  which  is  always  opposite  to  the  Sun's 
place.  The  form  of  the  Earth's  shadow  is  that  of  a  long,  sharply- 
pointed  cone,  which  has  our  globe  for  its  base.  Its  length,  varying 
with  the  distance  of  the  Earth  from  the  Sun,  is,  on  an  average,  855,- 
ooo  miles,  or  108  times  the  terrestrial  diameter.  This  conical  shadow 
of  the  Earth,  divided  longitudinally  by  the  plane  of  the  ecliptic,  lies 
half  above  and  half  below  that  plane,  on  which  the  summit  of  the 
shadow  describes  a  whole  circumference  in  the  course  of  a  year.  If 
the  Moon's  orbit  were  not  inclined  to  the  ecliptic,  our  satellite  would 
pass  at  every  Full  Moon  directly  through  the  Earth's  shadow  ;  but, 
owing  to  that  inclination?  it  usually  passes  above  or  below  the 
shadow.  Twice,  however,  during  each  of  its  revolutions,  it  must 
cross  the  plane  of  the  ecliptic,  the  points  of  its  orbit  where  this  hap- 
pens being  called  nodes.  Accordingly,  if  it  is  near  a  node  at  the 
time  of  Full  Moon,  it  will  enter  the  shadow  of  the  Earth,  and  be- 
come either  partly  or  wholly  obscured,  according  to  the  distance  of 
its  centre  from  the  plane  of  the  ecliptic.  The  partial  or  total 
obscuration  of  the  Moon's  disk  thus  produced  constitutes  a  partial 
or  total  eclipse  of  the  Moon.  The  essential  conditions  for  an 
eclipse  of  the  Moon  are,  therefore,  that  our  satellite  must  not  only 
be  full,  but  must  also  be  at  or  very  near  one  of  its  nodes. 

Although  inferior  in  importance  to  the  eclipses  of  the  Sun,  the 
eclipses  of  the  Moon  are,  nevertheless,  very  interesting  and  remark- 
able phenomena,  which  never  fail  to  produce  a  deep  impression  on 
the  mind  of  the  observer,  inasmuch  as  they  give  him  a  clear  insight 
into  the  silent  motions  of  the  planetary  bodies. 


54  THE    TROUVELOT 

At  the  mean  distance  of  the  Moon  from  the  Earth,  the  diameter 
of  the  conical  shadow  cast  in  space  by  our  globe  is  more  than 
twice  as  large  as  that  of  our  satellite.  But,  besides  this  pure  dark 
shadow  of  the  Earth,  its  cone  is  enveloped  by  a  partial  shadow 
called  "  Penumbra,"  which  is  produced  by  the  Sun's  light  being  par- 
tially, but  not  wholly,  cut  off  by  our  globe. 

While  the  Moon  is  passing  into  the  penumbra,  a  slight  reduction 
of  the  light  of  that  part  of  the  disk  which  has  entered  it,  is  noticea- 
ble. As  the  progress  of  the  Moon  continues,  the  reduction  becomes 
more  remarkable,  giving  the  impression  that  rare  and  invisible  va- 
pors are  passing  over  our  satellite.  Some  time  after,  a  small  dark 
indentation,  marking  the  instant  of  first  contact,  appears  on  the 
eastern  or  left-hand  border  of  the  Moon,  which  is  always  the  first  to 
encounter  the  Earth's  shadow,  since  our  satellite  is  moving  from 
west  to  east.  The  dark  indentation  slowly  and  gradually  en- 
larges with  the  onward  progress  of  the  Moon  into  the  Earth's 
shadow,  while  the  luminous  surface  of  its  disk  diminishes  in  the  same 
proportion.  The  form  of  the  Earth's  shadow  on  the  Moon's  disk 
clearly  indicates  the  rotundity  of  our  globe  by  its  circular  outline. 
Little  by  little  the  dark  segment  covers  the  Moon's  disk,  and  its 
crescent,  at  last  reduced  to  a  mere  thread  of  light,  disappears  at  the 
moment  of  the  second  contact.  With  this  the  phase  of  totality  be- 
gins, our  satellite  being  then  completely  involved  in  the  Earth's 
shadow. 

The  Moon  remains  so  eclipsed  for  a  period  of  time  which  varies 
with  its  distance  from  the  Earth,  and  with  the  point  of  its  orbit 
where  it  crosses  the  conical  shadow.  When  it  passes  through  the 
middle  of  this  shadow,  while  its  distance  from  our  globe  is  the  least, 
the  total  phase  of  an  eclipse  of  the  Moon  may  last  nearly  two  hours. 
The  left-hand  border  of  our  satellite  having  gone  first  into  the 
Earth's  shadow,  is  also  the  first  to  emerge,  and,  at  the  moment 
of  doing  so,  it  receives  the  Sun's  light,  and  totality  ends  with  the 
third  contact.  The  lunar  crescent  gradually  increases  in  breadth 
after  its  exit  from  the  shadow,  and  finally  the  Moon  recovers 
its  fully  illuminated  disk  as  before,  at  the  moment  its  western 
border  leaves  the  Earth's  shadow.  Soon  after,  it  passes  out  of  the 
penumbra,  and  the  eclipse  is  over.  In  total  eclipses,  the  interval  of 
time  from  the  first  to  last  contact  may  last  5h.  3Om,  but  it  is  usually 
shorter. 

Soon  after  the  beginning  of  an  eclipse,  the  dark  segment  pro- 
duced by  the  Earth's  shadow  on  the  Moon's  disk  generally  appears 
of  a  dark  grayish  opaque  color,  but  with  the  progress  of  the  phe- 


ASTRONOMICAL    DRAWINGS.  55 

nomenon,  this  dark  tint  is  changed  into  a  dull  reddish  color,  which, 
gradually  increasing,  attains  its  greatest  intensity  when  the  eclipse 
is  total.  At  that  moment  the  color  of  the  Moon  is  of  a  dusky,  red- 
dish, coppery  hue,  and  the  general  features  of  the  Moon's  surface 
are  visible  as  darker  and  lighter  tints  of  the  same  color.  It  some- 
times happens,  however,  that  our  satellite  does  not  exhibit  this 
peculiar  coppery  tint,  but  appears  either  blackish  or  bluish,  in 
which  case  it  is  hardly  distinguishable  from  the  sky. 

It  is  very  rare  for  the  Moon  to  disappear  completely  during  to- 
tality, and  even  when  involved  in  the  deepest  part  of  the  Earth's 
shadow,  our  satellite  usually  remains  visible  to  the  naked  eye,  or,  at 
least,  to  the  telescope.  This  phenomenon  is  to  be  attributed  to  the 
fact  that  the  portion  of  the  solar  rays  which  traverse  the  lower 
strata  of  our  atmosphere  are  strongly  refracted,  and  bend  inward  in 
such  a  manner  that  they  fall  on  the  Moon,  and  sufficiently  illumin- 
ate its  surface  to  make  it  visible.  The  reddish  color  observed  is 
caused  by  the  absorption  of  the  blue  rays  of  light  by  the  vapors 
which  ordinarily  saturate  the  lower  regions  of  our  atmosphere,  leav- 
ing only  red  rays  to  reach  the  Moon's  surface.  Of  course,  these 
phenomena  are  liable  to  vary  with  every  eclipse,  and  depend  almost 
exclusively  on  the  meteorological  conditions  of  our  atmosphere. 

In  some  cases  the  phase  of  totality  lasts  longer  than  it  should, 
according  to  calculation.  This  can  be  attributed  to  the  fact  that 
the  Earth  is  enveloped  in  a  dense  atmosphere,  in  which  opaque 
clouds  of  considerable  extent  are  often  forming  at  great  elevations. 
Such  strata  of  clouds,  in  intercepting  the  Sun's  light,  would  have, 
of  course,  the  effect  of  increasing  the  diameter  of  the  Earth's  shadow, 
in  a  direction  corresponding  to  the  place  they  occupy,  and,  if  the 
Moon  were  moving  in  this  direction,  would  increase  the  phase  of 
total  obscuration. 

The  eclipses  of  the  Moon,  like  those  of  the  Sun,  as  shown  above, 
have  a  cycle  of  18  years,  II  days  and  7  hours,  and  recur  after  this 
period  of  time  in  nearly  the  same  order.  They  can,  therefore,  be 
approximately  predicted  by  adding  i8y.  lid.  /h.  to  the  c}ate  of  the 
eclipses  which  have  occurred  during  the  preceding  period.  During 
this  cycle  70  eclipses  will  occur — 41  being  eclipses  of  the  Sun  and 
29  eclipses  of  the  Moon.  At  no  time  can  there  ever  be  more  than 
seven  eclipses  in  a  year,  and  there  are  never  less  than  two.  When 
there  are  only  two  eclipses  in  a  year,  they  are  both  eclipses  of  the 
Sun. 

Although  the  number  of  solar  eclipses  occurring  at  some  point 
or  other  of  the  Earth's  surface  is  greater  than  that  of  the  eclipses  of 


56  THE    TROUVELOT 

the  Moon,  yet  at  any  single  terrestrial  station  the  eclipses  of  the 
Moon  are  the  more  frequent.  While  an  eclipse  of  the  Sun  is  only 
visible  on  a  narrow  belt,  which  is  but  a  very  small  fraction  of  the 
hemisphere  then  illuminated  by  the  Sun,  an  eclipse  of  the  Moon  is 
visible  from  all  the  points  of  the  Earth  which  have  the  Moon  above 
their  horizon  at  the  time.  Furthermore,  an  eclipse  of  the  Sun  is  not 
visible  at  one  time  over  the  whole  length  of  its  narrow  tract,  but 
moves  gradually  from  one  end  of  it  to  the  other  ;  while,  on  the  con- 
trary, an  eclipse  of  the  Moon  begins  and  ends  at  the  very  same 
instant  for  all  places  from  which  it  can  be  seen,  but,  of  course, 
not  at  the  same  local  time,  which  varies  with  the  longitude  of  the 
place. 

The  partial  eclipse  of  the  Moon,  represented  on  Plate  VII.,  shows 
quite  plainly  the  configuration  of  our  satellite  as  seen  with  the 
naked  eye  during  the  eclipse,  with  its  bright  and  dark  spots,  and  its 
radiating  streaks.  This  eclipse  was  observed  on  October  24th,  1874. 


ASTRONOMICAL    DRA  WINGS.  57 


THE    PLANETS. 

AROUND  the  Sun  circulate  a  number  of  celestial  bodies,  which 
are  called  "  Planets"  The  planets  are  opaque  bodies,  and  appear 
luminous  because  their  surfaces  reflect  the  light  they  receive  from 
the  Sun. 

The  planets  are  situated  at  various  distances  from  the  Sun,  and 
revolve  around  this  body  in  widely  different  periods  of  time,  which 
are,  however,  constant  for  each  planet,  so  far  as  ascertained,  and 
doubtless  are  so  in  the  other  cases. 

The  ideal  line  traced  in  space  by  a  planet  in  going  around  the 
Sun,  is  called  the  orbit  of  the  planet ;  while  the  period  of  time  em- 
ployed by  a  planet  to  travel  over  its  entire  orbit  and  return  to  its 
starting  point,  is  called  the  sidereal  revolution,  or  year of  the  planet. 
The  dimensions  of  the  orbits  of  the  different  planets  necessarily  vary 
with  the  distance  of  these  bodies  from  the  Sun,  as  does  also  the 
length  of  their  sidereal  revolution. 

The  distance  of  a  planet  from  the  Sun  does  not  remain  con- 
stant, but  is  subject  to  variations,  which  in  certain  cases  are  quite 
large.  These  variations  result  from  the  fact  that  the  planetary 
orbits  are  not  perfect  circles  having  the  Sun  for  centre,  but  curves 
called  "Ellipses"  which  have  two  centres,  or  foci,  one  of  which  is 
always  occupied  by  the  Sun.  This  is  in  accordance  with  Kepler's 
first  law. 

The  ideal  point  situated  midway  between  the  two  foci  is  called 
the  centre  of  the  ellipse,  or  orbit ;  while  the  imaginary  straight  line 
which  passes  through  both  foci  and  the  centre,  with  its  ends  at 
opposite  points  of  the  ellipse,  is  called  "  the  major  axis  "  of  the  orbit. 
It  is  also  known  as  "  the  line  of  the  apsides"  The  ideal  straight  line 
which,  in  passing  through  the  centre  of  the  orbit,  cuts  the  major 
axis  at  right  angles,  and  is  prolonged  on  either  side  to  opposite 
points  on  the  ellipse,  is  called  "  the  minor  axis  "  of  the  orbit. 

When  a  planet  reaches  that  extremity  of  the  major  axis  of  its  or- 
bit which  is  the  nearest  to  the  Sun,  it  is  said  to  be  in  its  "perihelion  " 
while,  when  it  arrives  at  the  other  extremity,  which  is  farthest  from 
this  body,  it  is  said  to  be  in  its  "  aphelion"  When  a  planet  reaches 


58  THE    TROUVELOT 

either  of  the  two  opposite  points  of  its  orbit  situated  at  the  extrem- 
ities of  its  minor  axis,  it  is  said  to  be  at  its  mean  distance  from  the 
Sun. 

The  rapidity  with  which  the  planets  move  on  their  orbits  varies 
with  their  distance  from  the  Sun  ;  the  farther  they  are  from  this 
body,  the  more  slowly  they  move.  The  rapidity  of  their  motion  is 
greatest  when  they  are  in  perihelion,  and  least  when  they  are  in 
aphelion,  having  its  mean  rate  when  these  bodies  are  crossing  either 
of  the  extremities  of  the  minor  axes  of  their  orbits. 

The  imaginary  line  which  joins  the  Sun  to  a  planet  at  any  point 
of  its  orbit,  and  moves  with  this  planet  around  the  Sun,  is  called 
"  the  radius  vector  T  According  to  Kepler's  second  law,  whatever 
may  be  the  distance  of  a  planet  from  the  Sun,  the  radius  vector 
sweeps  over  equal  areas  of  the  plane  of  the  planet's  orbit  in  equal 
times. 

There  is  a  remarkable  relation  between  the  distance  of  the 
planets  from  the  Sun  and  their  period  of  revolution,  in  consequence 
of  which  the  squares  of  their  periodic  times  are  respectively  equal  to 
the  cubes  of  their  mean  distances  from  the  Sun.  From  this  third 
law  of  Kepler,  it  results  that  the  mere  knowledge  of  the  mean  dis- 
tance of  a  planet  from  the  Sun  enables  one  to  know  its  period  of 
revolution,  and  vice  versa. 

The  orbit  described  by  the  Earth  around  the  Sun  in  a  year,  or 
the  apparent  path  of  the  Sun  in  the  sky,  is  called  "  the  ecliptic''  Like 
that  of  all  the  planetary  orbits,  the  plane  of  the  ecliptic  passes 
through  the  Sun's  centre.  The  ecliptic  has  a  great  importance  in 
astronomy,  inasmuch  as  it  is  the  fundamental  plane  to  which  the 
orbits  and  motions  of  all  planets  are  referred. 

The  orbits  of  the  larger  planets  are  not  quite  parallel  to  the 
ecliptic,  but  more  or  less  inclined  to  this  plane  ;  although  the  incli- 
nation is  small,  and  does  not  exceed  eight  degrees.  On  account  of 
this  inclination  of  the  orbits,  the  planets,  in  accomplishing  their  rev- 
olutions around  the  Sun,  are  sometimes  above  and  sometimes  below 
the  plane  of  the  ecliptic.  A  belt  extending  8°  on  each  side  of  the 
ecliptic,  and,  therefore,  16°  in  width,  comprises  within  its  limits 
the  orbits  of  all  the  principal  planets.  This  belt  is  called  "the 
Zodiac? 

Since  all  the  planets  have  the  Sun  for  a  common  centre,  and  have 
their  orbits  inclined  to  the  ecliptic,  it  follows  that  each  of  these 
orbits  must  necessarily  intersect  the  plane  of  the  ecliptic  at  two  op- 
posite points  situated  at  the  extremities  of  a  straight  line  passing 
through  the  Sun's  centre.  The  two  opposite  points  on  a  planetary 


ASTRONOMICAL    DRA  WINGS.  59 

orbit  where  its  intersections  with  the  ecliptic  occur,  are  called  "  the 
Nodes"  and  the  imaginary  line  joining  them,  which  passes  through 
the  Sun's  centre,  is  called  "  the  line  of  the  nodes."  The  node  situ- 
ated at  the  point  where  a  planet  crosses  the  ecliptic  from  the  south 
to  the  north,  is  called  "the  ascending  node"  while  that  situated 
where  the  planet  crosses  from  north  to  south,  is  called  "  the  descend- 
ing node" 

The  planets  circulating  around  the  Sun  are  eight  in  number,  but, 
beside  these,  there  is  a  multitude  of  very  small  planets,  commonly 
called  "  asteroids,"  which  also  revolve  around  our  luminary.  The 
number  of  asteroids  at  present  known  surpasses  two  hundred,  and 
constantly  increases  by  new  discoveries.  In  their  order  of  distance 
from  the  Sun  the  principal  planets  are  :  Mercury,  Venus,  Earth, 
Mars,  Jupiter,  Saturn,  Uranus  and  Neptune.  The  orbits  of  the 
asteroids  are  comprised  between  the  orbits  of  Mars  and  Jupiter. 

When  the  principal  planets  are  considered  in  regard  to  their 
differences  in  size,  they  are  separated  into  two  distinct  groups  of 
four  planets  each,  viz.:  the  small  planets  and  the  large  planets.  The 
orbits  of  the  small  planets  are  wholly  within  the  region  occupied  by 
the  orbits  of  the  asteroids,  while  those  of  the  large  planets  are 
wholly  without  this  region. 

When  the  planets  are  considered  in  regard  to  their  position  with 
reference  to  the  Earth,  they  are  called  "  inferior  planets  "  and  "  su- 
perior planets."  The  inferior  planets  comprise  those  whose  orbits 
are  within  the  orbit  of  our  globe  ;  while  the  superior  planets  are 
those  whose  orbits  lie  beyond  the  orbit  of  the  Earth. 

Since  the  orbits  of  the  inferior  planets  lie  within  the  orbit  of  the 
Earth,  the  angular  distances  of  these  bodies  from  the  Sun,  as  seen 
from  the  Earth,  must  always  be  included  within  fixed  limits  ;  and 
these  planets  must  seem  to  oscillate  from  the  east  to  the  west, 
and  from  the  west  to  the  east  of  the  Sun  during  their  sidereal 
revolution.  In  this  process  of  oscillation  these  planets  some- 
times pass  between  the  Earth  and  the  Sun,  and  sometimes 
behind  the  Sun.  When  they  pass  between  us  and  the  Sun  they 
are  said  to  be  in  "inferior  conjunction,"  while,  when  they  pass 
behind  the  Sun,  they  are  said  to  be  in  "superior  conjunction." 
When  such  a  planet  reaches  its  greatest  distance,  either  east  or 
west,  it  is  said  to  be  at  its  greatest  elongation  east  or  west,  as  the 
case  may  be,  or  in  quadrature. 

The  superior  planets,  whose  orbits  lie  beyond  that  of  the  Earth 
and  enclose  it,  present  a  different  appearance.  A  superior  planet 
never  passes  between  the  Earth  and  the  Sun,  since  its  orbit  lies 


60  THE    TROUVELOT 

beyond  that  of  our  globe,  and,  therefore,  no  inferior  conjunction  of 
such  a  planet  can  ever  occur.  When  one  of  these  planets  passes 
beyond  the  Sun,  just  opposite  to  the  place  occupied  by  the  Earth, 
the  planet  is  said  to  be  in  "  conjunction  ;"  while,  when  it  is  on  the 
same  side  of  the  Sun  with  our  globe,  it  is  said  to  be  in  "  opposition." 
While  occupying  this  last  position,  the  planet  is  most  advantage- 
ously situated  for  observation,  since  it  is  then  nearer  to  the  Earth. 
The  period  comprised  between  two  successive  conjunctions,  or  two 
successive  oppositions  of  a  planet,  is  called  its  "synodical  period." 
This  period  differs  for  every  planet. 

It  is  supposed  that  all  the  planets  rotate  from  west  to  east,  like 
our  globe  ;  although  no  direct  evidence  of  the  rotation  of  Mercury 
Uranus,  and  Neptune  has  yet  been  obtained,  it  is  probable  that 
these  planets  rotate  like  the  others.  It  results  from  the  rotation  of 
the  planets  that  they  have  their  days  and  nights,  like  our  Earth, 
but  differing  in  duration  for  every  planet. 

The  axes  of  rotation  of  the  planets  are  more  or  less  inclined  to 
their  respective  orbits,  and  this  inclination  varies  but  little  in  the 
course  of  time.  From  the  inclination  of  the  axes  of  rotation  of  the 
planets  to  their  orbits,  it  results  that  these  bodies  have  seasons  like 
those  of  the  Earth  ;  but,  of  course,  they  differ  from  our  seasons  in 
duration  and  intensity,  according  to  the  period  of  revolution  and  the 
inclination  of  the  axis  of  each  separate  planet. 


ASTRONOMICAL     DRAWINGS.  61 


THE  PLANET  MARS. 

PLATE  VIII. 

MARS  is  the  fourth  of  the  planets  in  order  of  distance  from  the 
sun;  Mercury,  Venus  and  the  Earth  being  respectively  the  first, 
second  and  third. 

Owing  to  the  great  eccentricity  of  its  orbit,  the  distance  of  Mars 
from  the  Sun  is  subject  to  considerable  variations.  When  this  planet 
is  in  its  aphelion,  its  distance  from  the  Sun  is  152,000,000  miles,  but 
at  perihelion  it  is  only  126,000,000  miles  distant,  the  planet  being 
therefore  26,000,000  miles  nearer  the  Sun  at  perihelion  than  at 
aphelion.  The  mean  distance  of  Mars  from  the  Sun  is  139,000,- 
ooo  miles.  Light,  which  travels  at  the  rate  of  185,000  miles  a  second, 
occupies  12^2  minutes  in  passing  from  the  Sun  to  this  planet. 

While  the  distance  of  Mars  from  the  Sun  varies  considerably,  its 
distance  from  the  Earth  varies  still  more.  When  Mars  comes  into 
opposition,  its  distance  from  our  globe  is  comparatively  small,  espe- 
cially if  the  opposition  occurs  in  August,  as  the  two  planets  are  then 
as  near  together  as  it  is  possible  for  them  to  be,  their  distance  apart 
being  only  33,000,000  miles.  But  if  the  opposition  occurs  in  February, 
the  distance  may  be  nearly  twice  as  great,  or  62,000,000  miles.  On 
the  other  hand,  when  Mars  is  in  conjunction  in  August,  the  distance 
between  the  two  planets  is  the  greatest  possible,  or  no  less  than  245,- 
000,000  miles;  while,  when  the  conjunction  occurs  in  February,  it  is 
only  216,000,000  miles.  Hence  the  distance  between  Mars  and  the 
Earth  varies  from  33  to  245  millions  of  miles;  that  is,  this  planet 
may  be  212  million  miles  nearer  to  us  at  its  nearest  oppositions  than 
at  its  most  distant  conjunctions. 

From  these  varying  distances  of  Mars  from  the  Earth,  necessarily 
result  great  variations  in  the  brightness  and  apparent  size  of  the 
planet,  as  seen  from  our  globe.  When  nearest  to  us  it  is  a  very  con- 
spicuous object,  appearing  as  a  star  of  the  first  magnitude,  and 
approaching  Jupiter  in  brightness;  but  when  it  is  farthest  it  is  much 


62  THE    TROUVELOT 

reduced,  and  is  hardly  distinguishable  from  the  stars  of  the  second 
and  even  third  magnitude.  In  the  first  position,  the  apparent  diam- 
eter of  Mars  is  26",  in  the  last  it  is  reduced  to  3"  only. 

The  orbit  of  Mars  has  the  very  small  inclination  of  i°  51'  to  the 
plane  of  the  ecliptic.  The  planet  revolves  around  the  Sun  in  a 
period  of  687  days,  which  constitutes  its  sidereal  year,  the  year  of 
Mars  being  only  43  days  less  than  two  of  our  years. 

Mars  travels  along  its  orbit  with  a  mean  velocity  of  15  miles  per 
second,  being  about  -fV  of  the  velocity  of  our  globe  in  its  orbit.  The 
synodical  period  of  Mars  is  2  years  and  48  days,  during  which  the 
planet  passes  through  all  its  degrees  of  brightness. 

Mars  is  a  smaller  planet  than  the  Earth,  its  diameter  being  only 
4,200  miles,  and  its  circumference  13,200  miles.  It  seems  well  estab- 
lished that  it  is  a  little  flattened  at  its  poles,  but  the  actual  amount 
of  this  flattening  is  difficult  to  obtain.  According  to  Prof.  Young, 
the  polar  compression  is  -5-^-. 

The  surface  of  this  planet  is  a  little  over  y2^  of  the  surface  of  our 
globe,  and  its  volume  is  6^4  times  less  than  that  of  the  Earth.  Its 
mass  is  only  about  TV,  while  its  density  is  about  ^  that  of  the 
Earth.  The  force  of  gravitation  at  its  surface  is  nearly  ^  of  what 
it  is  at  the  surface  of  our  globe. 

The  planet  Mars  rotates  on  an  axis  inclined  61°  18'  to  the  plane 
of  its  orbit,  so  that  its  equator  makes  an  angle  of  28°  42'  with  the 
same  plane.  The  period  of  rotation  of  this  planet,  which  constitutes 
its  sidereal  day,  is  24  h.  37  m.  23  s. 

The  year  of  Mars,  which  is  composed  of  669^3  of  these  Martial 
days,  equals  687  of  our  days,  this  planet  rotating  669^3  times  upon 
Its  axis  during  this  period.  But  owing  to  the  movement  of  Mars 
around  the  Sun,  the  number  of  solar  days  in  the  Martial  year  is  only 
6682/3,  while,  owing  to  the  same  cause,  the  solar  day  of  Mars  is  a  lit- 
tle longer  than  its  sidereal  day,  and  equals  24  h.  39  m.  35  s. 

The  days  and  nights  on  Mars  are  accordingly  nearly  of  the 
same  length  as  our  days  and  nights,  the  difference  being  a  little 
less  than  three-quarters  of  an  hour.  But  while  the  days  and  nights 
of  Mars  are  essentially  the  same  as  ours,  its  seasons  are  almost  twice 
as  long  as  those  of  the  Earth.  Their  duration  for  the  northern  hemi- 
sphere, expressed  in  Martial  days,  is  as  follows:  Spring,  191;  Sum- 
mer, 181;  Autumn,  149;  Winter,  147.  While  the  Spring  and  Sum- 
mer of  the  northern  hemisphere  together  last  372  days,  the  Autumn 
and  Winter  of  the  same  hemisphere  last  only  296  days,  or  76  days 
less.  Since  the  summer  seasons  of  the  northern  hemisphere  corre- 
spond to  the  winter  seasons  of  the  southern  hemisphere,  and  vice 


ASTRONOMICAL     DRAWINGS.  63 

versa,  the  northern  hemisphere,  owing  to  its  longer  summer,  must 
accumulate  a  larger  quantity  of  heat  than  the  last.  But  on  Mars,  as 
on  the  Earth,  there  is  a  certain  law  of  compensation  resulting  from 
the  eccentricity  of  the  planet's  orbit,  and  from  the  fact  that  the  mid- 
dle of  the  summer  of  the  southern  hemisphere  of  this  planet,  coin- 
cides with  its  perihelion.  From  the  greater  proximity  of  Mars  to 
the  Sun  at  that  time,  the  southern  hemisphere  then  receives  more 
heat  in  a  given  time  than  does  the  northern  hemisphere  in  its  summer 
season.  When  everything  is  taken  into  account,  however,  it  is  found 
that  the  southern  hemisphere  must  have  warmer  summers  and  colder 
winters  than  the  northern  hemisphere. 

Seen  with  the  naked  eye,  Mars  appears  as  a  fiery  red  star,  whose 
intensity  of  color  is  surpassed  by  no  other  star  in  the  heavens.  Seen 
through  the  telescope,  it  retains  the  same  red  tint,  which,  however, 
appears  less  intense,  and  gradually  fades  away  toward  the  limb, 
where  it  is  replaced  by  a  white  luminous  ring. 

Mars  is  a  very  difficult  object  to  observe,  the  atmosphere  sur- 
rounding it  being  sometimes  so  cloudy  and  foggy  that  the  sight  can 
hardly  penetrate  through  its  vapors.  When  this  planet  is  observed 
under  favorable  atmospheric  conditions,  and  with  sufficient  mag- 
nifying power,  its  surface,  which  is  of  a  general  reddish  tint,  is  found 
to  be  diversified  by  white,  gray  and  dark  markings.  The  dark 
markings,  which  are  the  most  conspicuous,  almost  completely  sur- 
round the  planet.  They  are  of  different  forms  and  sizes,  and  very 
irregular,  as  can  be  seen  on  Plate  VIII.,  which  represents  one  of  the 
hemispheres  of  this  planet.  Many  of  them,  especially  those  situated 
in  the  tropical  regions  of  the  planet,  form  long  narrow  bands,  whose 
direction  is  in  the  main  parallel  to  the  Martial  equator. 

The  dusky  spots  differ  very  much,  both  from  one  another  and  in 
their  several  parts,  as  regards  intensity  of  shade.  Some  appear 
almost  black,  while  others  which  appear  grayish,  are  so  faint,  that 
they  can  seldom  be  seen.  In  the  southern  hemisphere,  the  darkest 
part  of  the  spots  is  generally  found  along  their  northern  border; 
especially  where  there  are  deep  indentations. 

Some  observers  have  described  these  spots  as  being  greenish  or 
bluish,  but  I  have  never  been  able  to  see  the  faintest  trace  of  these 
colors  in  them,  except  when  they  were  observed  close  to  the  limb, 
and  involved  in  the  greenish  tinted  ring  which  is  always  to  be  seen 
there.  It  is  probably  an  effect  of  contrast,  since  green  and  red  are 
complementary  colors,  and  since  this  greenish  tinge  around  the 
limb  covers  all  kinds  of  spots,  whether  white  or  dark.  When  such 
dark  spots,  involved  in  the  greenish  tint,  are  carried  by  the  rotation 


64  THE    TROUVELOT 

towards  the  centre  of  the  disk,  they  no  longer  show  this  greenish 
color.  To  me,  these  spots  have  always  appeared  dark,  and  of  such 
tints  as  would  result  from  a  mixture  of  white  and  black  in  different 
proportions  ;  except  that  on  their  lighter  portions  they  show  some  of 
the  prevalent  reddish  tint  of  the  Martial  surface.  It  is  to  be  remarked 
that  in  moments  of  superior  definition  of  the  telescopic  image,  the 
intensity  of  darkness  of  all  the  spots  is  considerably  increased — some 
of  them  appearing  almost  perfectly  black. 

The  markings  on  the  surface  of  Mars  are  now  tolerably  well 
known — especially  those  of  its  southern  hemisphere,  which,  owing  to 
the  greater  proximity  of  the  planet  to  our  globe  when  this  hemis- 
phere is  inclined  towards  the  earth,  have  been  better  studied.  Those 
of  the  northern  hemisphere  are  not  so  well  known,  since  when  this 
hemisphere  is  inclined  towards  us,  the  distance  of  Mars  from  the 
Earth  is  26,000,0x30  miles  greater,  so  that  the  occasions  for  observing 
them  are  not  so  favorable. 

Several  charts  of  Mars  are  in  existence,  but  as  the  same  nomen- 
clature has  not  been  employed  in  all  of  them,  some  confusion  has 
arisen  in  regard  to  the  names  given  to  the  most  remarkable  features 
of  trie  planet's  surface.  In  order  to  give  clearness  to  the  subject,  it 
will  be  necessary  here  to  give  a  brief  description  of  the  principal 
markings  represented  on  Plate  VIII.  In  this  the  nomenclature  will 
be  employed  which  has  been  adopted  by  the  English  observers  in  the 
fine  chart  of  Mr.  Nath.  Green.  The  large  dark  spot  represented  on 
the  left-hand  side  of  the  plate  is  called  De  La  Rue  Ocean.  The  dark 
oval  spot,  isolated  in  the  vicinity  of  the  centre  of  the  disk,  is  called 
Terby  Sea  ;  while  the  dark,  irregular  form  on  the  right,  near  the 
border,  represents  the  western  extremity  of  Maraldi  Sea. 

The  dusky  spots  of  Mars  seem  to  be  permanent,  and  to  form  a 
part  of  the  general  surface  of  the  planet.  That  several  among  them, 
at  least,  are  permanent,  is  proved  by  the  fact  that  they  have  been 
observed  in  the  same  position,  and  with  the  same  general  form,  for 
over  two  centuries.  Yet,  if  we  are  to  depend  upon  the  drawings 
made  fifty  years  ago  by  Beer  and  Maedler,  it  would  seem  that  the 
permanency  of  some  of  them  does  not  exist,  since  a  very  large  spot 
represented  by  these  astronomers  on  their  chart  of  Mars  is  not  visi- 
ble now.  This  object,  which,  on  their  map,  has  its  middle  at  270°, 
should  be  precisely  under  the  prominent  dark  oval  spot  called  Terby 
Sea,  seen  near  the  centre  of  the  picture,  and  would  extend  down 
almost  as  far  as  the  northern  limb.  This  can  hardly  be  attributed 
to  an  error  of  observation,  since  these  observers  were  both  careful, 
and  had  great  experience  in  this  class  of  work.  It  is  a  very  singu- 


ASTRONOMICAL    DRAWINGS.  65 

lar  fact  that,  at  the  very  same  place  where  Maedler  represented  the 
spot  in  question,  I  found  a  conspicuous  dark  mark  on  December 
1 6,  1 88 1,  which  was  certainly  not  visible  in  1877,  during  one  of 
the  most  favorable  oppositions  which  can  ever  occur.  The  object, 
which  is  still  visible  (Feb.,  1882),  consists  of  an  isolated  spot  situ- 
ated a  little  to  the  north  of  Terby  Sea.  During  the  memorable  op- 
position of  1877,  I  investigated  thoroughly  the  markings  of  Mars,  and 
made  over  200  drawings  of  its  disk,  32  of  which  represent  the  Terby 
Sea;  but  this  isolated  spot  was  not  to  be  seen,  unless  it  be  identified 
with  the  faint  mark,  represented  on  the  plate,  which  occupied  its 
place.  There  cannot  be  the  slightest  doubt  that  a  change  has  oc- 
curred at  that  place.  Changes  in  the  markings  have  also  been  sus- 
pected on  the  other  hemisphere  of  the  planet. 

The  well-known  fact  that  the  continents,  and  especially  the 
mountainous  and  denuded  districts  of  our  globe,  reflect  much  more 
light  than  the  surfaces  covered  by  water,  has  led  astronomers  to  sup- 
pose that  the  dark  spots  on  Mars  are  produced  by  a  liquid  strongly 
absorbing  the  rays  of  light,  like  the  liquids  on  the  surface  of  the 
Earth.  According  to  this  theory  the  dark  spots  observed  are  sup- 
posed to  be  lakes,  seas,  and  oceans,  similar  to  our  own  seas  and 
oceans,  while  the  reddish  and  whitish  surfaces  separating  these  dark 
spots,  are  supposed  to  be  islands,  peninsulas,  and  continents.  This 
supposition  seems  certainly  to  have  a  great  deal  of  probability  in  its 
favor,  although  some  of  the  lighter  markings  may  have  a  different 
origin,  and  perhaps  be  due  to  vegetation  ;  but  no  observer  has  yet 
seen  in  them  any  of  the  changes  which  ought  to  result  from  change 
of  seasons.  Some  of  the  changes  in  the  dark  spots  might  also  be 
'  attributed  to  the  flooding  or  drying  up  of  marshes  and  low  land. 
The  change  which  I  have  observed  lately  might  be  attributed  to 
such  a  cause,  especially  as  my  observation  was  made  shortly  after 
the  spring  equinox  of  the  northern  hemisphere  of  Mars,  which  oc- 
curred on  December  8th. 

Besides  the  dark  spots  just  described,  there  are  markings  of  a  dif- 
ferent character  and  appearance.  Among  the  most  conspicuous  are 
two  very  brilliant  white  oval  spots,  which  always  occupy  opposite 
sides  of  the  planet.  These  two  bright  spots,  which  correspond  very 
closely  with  the  poles  of  rotation  of  Mars,  have  been  called  "  polar 
spots." 

On  account  of  the  inclination  of  the  axis  of  rotation  of  this  planet 
to  the  ecliptic,  it  is  rare  that  both  of  these  spots  are  visible  on  the 
disk  at  the  same  time  ;  and  when  this  occurs,  they  are  seen  consid- 
erably foreshortened,  as  they  are  then  both  on  the  limb  of  the  planet. 


66  THE    TROUVELOT 

Usually  only  one  spot  is  visible,  and  it  appears  to  its  best  advan- 
tage when  the  region  to  which  it  belongs  attains  its  maximum  of  in- 
clination towards  the  Earth. 

The  polar  spots  change  considerably  in  size,  as  they  do  also  in 
form.  Sometimes  they  occupy  nearly  one-third  of  the. disk,  as  is 
proved  by  many  of  my  observations  ;  while  at  other  times  they  are 
so  much  reduced  as  to  be  totally  invisible.  It  is  to  be  remarked  that 
the  reduction  of  these  spots  generally  corresponds  with  the  summer 
seasons,  and  their  enlargement  with  the  winter  seasons  of  the  hemi- 
spheres to  which  they  respectively  belong.  From  these  well-observed 
facts  it  would  appear  that  a  relation  exists  between  the  temperature 
of  the  two  hemispheres  of  Mars  and  the  variations  of  the  white  spots 
observed  at  its  poles.  A  similar  relation  is  known  to  exist  on  our 
globe  between  the  progress  of  the  seasons  and  the  melting  away  and 
the  accumulation  of  snow  in  the  polar  regions.  Astronomers  have 
been  led,  accordingly,  to  attribute  the  polar  spots  of  Mars,  with  all 
their  variations,  to  the  alternate  accumulation  and  melting  of  snows. 
On  this  account,  the  polar  spots  of  Mars  are  sometimes  also  called 
"  snow-spots." 

Errors  have  certainly  been  made  by  astronomers  in  some  of  their 
observations  of  the  so-called  polar  snow-spots,  other  objects  occu- 
pying their  place  having  been  mistaken  for  them.  A  regular  series 
of  observations  on  this  planet,  which  I  have  now  continued  for  seven 
years,  has  revealed  the  fact  that  during  the  winter  seasons  of  the 
southern  hemisphere  of  Mars,  the  polar  spots  are  most  of  the  time 
invisible,  being  covered  over  by  white,  opaque,  cloud-like  forms, 
strongly  reflecting  light.  In  1877,  during  more  than  a  month,  I, 
myself,  mistook  for  the  polar  spots  such  a  canopy  of  clouds,  which 
covered  at  least  one-fifth  of  the  surface  of  the  whole  disk.  I  only 
became  aware  of  my  error  when  the  opaque  cloud,  beginning  to  dis- 
solve at  the  approach  of  the  Martial  summer,  allowed  the  real  polar 
spot  to  be  seen  through  its  vapors,  as  through  a  mist  at  first,  and 
afterwards  with  great  distinctness.  In  this  particular  case,  the  snow- 
spot  was  considerably  smaller  than  the  cloudy  cap  which  covered  it, 
and  it  is  to  be  remarked  that  it  was  not  situated  at  the  centre  of  this 
cloudy  cap,  but  was  east  of  that  centre  ;  a  fact  which  may  account 
for  the  so-called  polar  spots  not  being  always  observed  on  exactly 
opposite  sides  of  the  disk.  From  my  observations  of  1877,  1878  and 
1880,  it  appears  that  at  the  approach  of  the  autumnal  equinox  of  the 
southern  hemisphere  of  Mars,  large,  opaque  masses,  like  cumulus  clouds 
in  form,  began  to  gather  in  the  polar  regions  of  that  hemisphere,  and 
continued  through  autumn  and  winter,  dissolving  only  at  the  ap- 


ASTRONOMICAL    DRAWINGS,  67 

proach  of  spring.  These  clouds,  which  varied  in  form  and  extent, 
were  very  unsteady  at  first,  but  as  the  winter  drew  nearer  they 
enlarged  and  became  more  permanent,  covering  large  surfaces  for 
months  at  a  time. 

That  the  large  white  spots  under  consideration  are  real  clouds  in 
the  atmosphere  of  Mars,  and  are  not  due  to  a  fall  of  snow,  is  proved 
by  the  fact  that  these  spots  covered  both  seas  and  continents  with 
equal  facility,  even  in  the  equatorial  regions  of  the  planet.  Snow, 
of  course,  could  not  cover  the  seas  of  Mars,  unless  these  were  all 
frozen  over,  even  in  the  equatorial  zones  ;  therefore,  if  the  dark  spots 
of  Mars  are  assumed  to  be  due  to  water,  these  large  white  spots  can- 
not well  be  ascribed  to  snow. 

The  real  polar  spots  of  Mars  seem  to  be  in  relief  on  the  surface 
of  the  planet,  since  the  southern  spot  often  appeared  slightly  shaded 
on  the  side  opposite  to  the  Sun  during  my  observations  in  1877.  In 
certain  cases,  when  they  are  on  or  very  near  the  limb,  they  have 
been  observed,  both  by  others  and  by  myself,  to  project  from  the 
disk  slightly. 

The  polar  spots  of  Mars  are  doubtless  composed  of  a  material 
which,  like  our  snow  or  ice,  melts  under  the  rays  of  the  Sun  ;  although 
it  seems  difficult  to  admit  that  the  Martial  snow  is  identical  with  our 
terrestrial  snow,  and  that  it  melts  at  a  like  temperature.  The  south 
polar  spot  of  Mars  entirely  disappeared  from  sight  in  its  summer  sea- 
son in  1877,  although  the  planet  receives  less  than  one-half  as  much 
heat  as  we  receive  from  the  Sun  ;  yet  on  our  globe  the  arctic  or  ant- 
arctic ices  and  snow  are  perpetual — never  melting  entirely.  An  im- 
portant fact  disclosed  by  the  melting  away  of  the  southern  polar 
spot  is,  that  in  melting  it  is  always  surrounded  with  a  very  dark  sur- 
face, which  takes  the  place  of  the  melted  portion  of  the  spot,  as 
observed  by  myself  in  i877~'78.  When  the  polar  spot  had  entirely 
disappeared,  its  place  was  occupied  by  a  very  dark  spot.  Now,  if 
the  polar  spot  is  really  ice,  and  the  dark  spots  are  actual  seas,  this 
polar  spot  must  be  situated  in  mid-ocean,  since,  on  melting  away,  it 
is  replaced  by  a  dark  spot.  If  the  polar  spots  are  composed  of  a  white 
substance  melting  under  the  rays  of  the  Sun,  as  seems  altogether 
probable,  its  melting  point  must  be  above  that  of  terrestrial  snow. 

Many  of  the  dark  spots  of  Mars,  and  especially  those  whose 
northern  border  forms  an  irregular  belt  upon  the  equatorial  regions 
of  this  planet,  are  bordered  on  that  side  by  a  white  luminous  belt, 
following  all  their  sinuosities.  These  white  borders  are  variable. 
Sometimes  they  are  very  prominent  and  intensely  bright,  especially 
at  some  points,  which  occasionally  almost  equal  the  polar  spots  in 


68  THE     TROUVELOT 

brilliancy;  while  at  other  times  they  are  so  faint,  that  they  can 
hardly  be  distinguished,  or  are  even  invisible;  although  the  atmos- 
phere is  clear  and  the  dark  spots  appear  perfectly  well  defined. 
While  these  white  borders  were  invisible,  I  have  sometimes  watched 
for  several  hours  at  a  time  to  see  if  I  could  detect  any  traces  of  them 
in  places  where  they  usually  appear  the  most  prominent,  but  gen- 
erally without  success.  On  a  few  occasions,  however,  I  had  the 
good  fortune  to  see  some  of  these  spots  forming  gradually  in  the 
course  of  one  or  two  hours,  at  places  where  nothing  of  them  could 
be  seen  before. 

These  whitish  fringes  forming  and  vanishing  along  the  coasts  of  the 
Martial  seas  have  been  very  little  studied  by  astronomers.  From 
my  observations  made  during  the  last  seven  years,  it  appears  very 
probable  that  this  belt  and  its  white  spots  are  mainly  due  to  the 
condensation  of  vapors  around,  and  over  high  peaks,  and  extensive 
mountain  chains,  forming  the  Martial  sea-coasts,  as  the  Andes  and 
Rocky  Mountains  form  the  sea-coasts  of  the  Pacific  Ocean.  These 
high  mountains  on  Mars,  condensing  the  vapors  into  fogs  or  clouds 
above  them,  or  at  their  sides,  as  often  happens  in  our  mountainous 
districts,  would  certainly  suffice  to  produce  the  phenomena  observed. 
Some  of  the  highest  peaks  among  these  mountain  chains  may  even 
have  their  summits  covered  with  perpetual  snow,  or  some  substance 
partaking  of  the  nature  of  snow.  The  temporary  visibility  and  in- 
visibility of  the  white  spots  seen  on  Mars,  as  well  as  the  rapid  trans- 
formations they  sometimes  undergo,  may  be  explained  as  caused  by 
clouds  having  a  high  reflective  power  and  a  liability  to  form  and 
disappear  quickly. 

The  assumption  that  these  irregular  whitish  bands  and  spots  are 
formed  by  the  condensation  of  vapors  on  mountain  chains,  and  ele- 
vated table  lands,  is  supported  by  my  observations  made  in  1877 
and  1879.  When  such  white  spots  were  traversing  the  terminator 
at  sunrise,  they  very  often  projected  far  into  the  night  side,  thus  in- 
dicating that  they  were  at  a  higher  level  than  that  of  the  general 
surface.  Indentations  in  the  terminator,  corresponding  to  large 
dark  spots  crossing  its  line,  also  clearly  indicated  the  depression  of 
the  dark  spots  below  the  general  surface.  The  highest  mountain- 
ous districts  thus  observed  on  Mars,  are  situated  between  60°  and 
70°  of  south  latitude,  towards  the  western  extremity  of  Gill  Land. 
The  mountain  chain,  which  almost  completely  forms  the  surface  of 
this  land,  is  so  elevated  at  some  points,  that  they  not  only  change 
the  form  of  the  terminator  when  they  are  seen  upon  it,  but  also 
the  limb  of  the  planet,  as  seen  by  myself.  They  then  appear  so 


ASTRONOMICAL    DRAWINGS.  69 

brilliant,  that  the  principal  summit  among  them  has  been  mis- 
taken by  several  observers  for  the  polar  spot  itself,  as  proved  by 
the  wrong  position  assigned  to  it  on  their  drawings.  It  seems  prob- 
able that  this  high  peak,  which  appears  always  white,  is  constantly 
covered  with  snow,  or  the  similar  material  replacing  it  on  Mars. 
This  high  region  is  situated  between  longitudes  180°  and  190°. 

The  highest  mountainous  parts  belonging  to  the  hemisphere  rep- 
resented on  Plate  VIII.,  which  are  nearly  always  more  or  less  visible 
as  whitish  spots  and  bands,  form  a  coast  line  along  the  northern 
(lower)  border  of  De  La  Rue  Ocean.  This  great  spot,  which  is  not 
so  simple  as  it  has  been  represented  by  observers,  is  in  fact  divided 
by  two  narrow  isthmuses,  one  in  the  north,  the  other  in  the  east, 
both  joining,  in  the  interior  of  the  great  ocean,  a  peninsula  hereto- 
fore known  as  Hall  Island.  Upon  the  south-eastern  extremity  of 
this  peninsula,  a  white  spot,  called  Dawes  Ice  Island,  was  observed 
in  1865,  but  it  soon  disappeared,  and  was  after  that  seen  only  now 
and  then.  It  is  very  probable  that  this  so-called  Ice  Island  was 
due  to  clouds  forming  around  the  summit  of  some  high  peak  of  this 
peninsula. 

On  the  opposite  hemisphere  to  that  represented  on  Plate  VIII., 
the  white  fringes  bordering  the  dark  spots  are  much  more  con- 
spicuous than  they  are  on  this  side.  On  the  eastern  side  of  a  re- 
markable dark  spot  called  Kaiser  Sea,  they  are  very  bright,  and 
almost  always  present,  although  they  vary  considerably,  both  in 
brightness  and  in  extent.  To  the  south  of  Kaiser  Sea,  they  are  very 
conspicuous  on  the  eastern  border  of  Lockyer  Land,  forming  an  ele- 
vated and  deeply  indented  coast-line  along  Lambert  Sea.  There 
the  white  spots  never  disappear  entirely,  being  always  visible  on  the 
north  side,  where  they  turn  westward  along  Dawes'  Ocean — the 
mountain  chain  attaining  there  its  greatest  altitude.  Very  frequently 
Lockyer  Land,  which  seems  to  be  a  vast  plateau,  appears  throughout 
white  and  brilliant,  this  occurring  usually  towards  the  sunrise  or  sun- 
set of  that  region,  probably  from  the  condensation  of  vapors  and  the 
formation  of  fogs,  but  generally  this  whiteness  gradually  disappears 
with  the  progress  of  the  sun  above  this  plateau.  Inside  of  the  great 
continents  of  Mars  these  temporary  white  spots  are  not  so  frequent, 
but  when  visible  they  occupy  always  the  same  positions — a  fact 
which  probably  indicates  that  they  occupy  the  culminating  points  of 
these  continents.  One  of  these  temporary  white  spots  inside  of  the 
continents  is  represented  on  Plate  VIII.,  on  the  left-hand  side,  below 
De  La  Rue  Ocean,  on  Maedler  Continent. 

Although  large,  opaque,  cumulus-shaped,  cloud-like  forms  are  seen 


70  THE    TROUVELOT 

in  the  polar  regions  of  Mars,  such  forms  are  very  seldom  seen  in  the 
tropical  zones,  or,  at  least,  it  appears  so,  from  the  fact  that  my  observ- 
ations, continued  during  the  last  seven  years,  have  disclosed  no  real 
opaque  cloudy  forms  there.  Although  the  Martial  sky  is  frequently 
overcast  by  dense  vapors  or  thick  fogs  in  these  regions,  yet  no 
real  opaque  clouds  were  ever  seen  ;  the  most  prominent  among  the 
dusky  spots  being  faintly  visible  through  the  vapory  veil,  when 
they  approached  the  centre  of  the  disk. 

Besides  these  phenomena,  which  prove  that  Mars  is  surrounded 
by  an  atmosphere  having  a  great  deal  of  similarity  to  our  own,  a 
further  proof  is  afforded  by  the  fact  that  the  dark  spots,  which  appear 
sharply  defined  and  black  when  they  are  seen  near  the  centre,  be- 
come less  and  less  visible  as  they  advance  towards  the  limb,  and  are 
totally  invisible  before  they  reach  it.  Moreover,  the  spectroscope 
also  indicates  the  existence  of  an  atmosphere,  and  even  the  presence 
of  watery  vapor  in  it.  A  very  curious  state  of  the  Martial  atmos- 
phere is  revealed  by  my  observations  of  1877-78.  During  eight  con- 
secutive weeks,  from  December  I2th  to  February  6th,  a  whole  hem- 
isphere of  the  planet — precisely  that  represented  on  Plate  VIII.— 
was  completely  covered  by  dense  vapors,  or  a  thick  fog  which  barely 
allowed  the  dark  spots  to  be  seen  through  it,  even  when  they  were 
in  the  centre  of  the  disk.  The  opposite  hemisphere  of  Mars  appeared 
just  as  clear  and  calm  as  possible  ;  there  all  the  spots  and  their  mi- 
nutest details  could  be  seen,  and  when  the  planet  was  observed  at  the 
proper  time,  the  line  separating  the  foggy  from  the  clear  side  was 
plainly  visible. 

The  reddish  tint  observed  on  the  continents  of  Mars  has  been 
supposed  by  some  astronomers  to  be  the  real  color  of  the  atmos- 
phere of  this  planet.  But,  for  many  reasons,  this  explanation  is  not 
acceptable.  Besides  the  fact  that  the  border  of  the  planet  appears 
white,  while  it  should  be  more  red  than  the  other  part,  owing  to  the 
greater  depth  of  atmosphere  there  presented  to  us,  the  polar  spots, 
the  white  bands  along  the  sea-coasts,  and  the  cloud-like  forms  ap- 
pear perfectly  white,  not  the  slightest  tint  of  red  being  visible  on 
them,  as  would  be  the  case  if  these  objects  were  seen  through  an 
atmosphere  tinted  red.  Other  astronomers  have  supposed  that  the 
vegetation  of  this  planet  has  a  reddish  color  ;  but  this  is  not  sup- 
ported by  observation.  It  has  been  again  supposed,  with  much  more 
probability,  that  the  surface  of  Mars  is  composed  of  an  ochreous  mate- 
rial which  gives  the  planet  its  predominant  ruddy  color. 

Until  lately  Mars  was  supposed  to  be  without  a  satellite,  but  in 
August,  1877,  Professor  Hall,  of  the  Washington  Observatory,  made 


ASTRONOMICAL    DRA  WINGS.  71 

one  of  the  most  remarkable  discoveries  of  the  time,  and  found  two 
satellites  revolving  around  this  planet.  These  satellites  are  among 
the  smallest  known  heavenly  bodies,  their  diameter  having  been  esti- 
mated at  from  6  to  10  miles  for  the  outer  satellite,  and  from  10  to 
40  miles  for  the  inner  one. 

The  most  extraordinary  feature  of  these  bodies  is  the  proximity 
of  the  inner  satellite  to  the  planet,  and  the  consequent  rapidity  of 
its  motion.  The  distance  of  the  inner  satellite  from  the  centre  of 
Mars  is  about  6,000  miles,  and  from  surface  to  surface  it  is  less  than 
4,000  miles,  or  a  little  more  than  the  distance  from  New  York  to 
San  Francisco.  The  shortest  period  of  revolution  of  any  satel- 
lite previously  known,  is  that  of  the  inner  satellite  of  Saturn,  which 
is  a  little  more  than  22^2  hours  ;  but  the  inner  satellite  of  Mars  ac- 
complishes its  revolution  in  /h.  38m.,  or  in  17  hours  less  than  the 
period  of  rotation  of  the  planet  upon  its  axis.  The  period  of  revo- 
lution of  the  outer  satellite  is  greater,  of  course,  and  equals  3Oh.  /m. 

From  this  rapidity  of  motion  of  the  inner  satellite  of  Mars,  a  very 
curious  result  follows,  which  at  first  sight  may  appear  in  contradic- 
tion with  the  fact  that  this  body  has  a  direct  motion,  like  that  of  all 
the  planets  of  the  solar  system,  and  moves  around  Mars  from  west 
to  east.  While  the  outer  satellite  of  this  planet,  in  company  with 
all  the  stars  and  planets,  rises  in  the  east  and  sets  in  the  west,  the 
inner  satellite,  on  the  contrary,  rises  in  the  west  and  sets  in  the 
east.  Since  the  period  of  rotation  of  Mars  is  greater  than  is  the 
period  of  revolution  of  this  satellite,  it  necessarily  follows  that  this 
last  body  must  constantly  be  gaining  on  the  rotation,  and,  conse- 
quently, that  the  satellite  sets  in  the  east  and  rises  in  the  west, 
compassing  the  whole  heavens  around  Mars  three  times  a  day,  pass- 
ing through  all  its  phases  in  1 1  hours,  each  quarter  of  this  singular 
Moon  lasting  less  than  3  hours. 

It  has  been  shown  above  that  Mars  has  many  points  of  resem- 
blance to  the  Earth.  It  has  an  atmosphere  constituted  very  nearly 
like  ours  ;  it  has  fogs,  clouds,  rains,  snows,  and  winds.  It  has  water, 
or  at  least  some  liquids  resembling  it  ;  it  has  rivers,  lakes,  seas 
and  oceans.  It  has  also  islands,  peninsulas,  continents,  mountains 
and  valleys.  It  has  two  Moons,  which  must  create  great  and  rapid 
tides  in  the  waters  of  its  seas  and  oceans.  It  has  its  days  and  nights, 
its  warm  and  cold  seasons,  and  very  likely  its  vegetation,  its  prairies 
and  forests,  like  the  Earth.  On  the  other  hand,  its  year  and  sea- 
sons are  double  those  of  the  Earth,  and  its  distance  from  the  Sun 
is  greater. 

Is  this  planet,  which  is  certainly  constituted  very  nearly  like  our 


72 


THE    IROUVELOT 


globe,  and  seems  so  nearly  fitted  for  the  wants  of  the  human  race, 
inhabited  by  animals  and  intelligent  beings  ? 

To  answer  this  question,  either  in  the  negative  or  in  the  affirma- 
tive, would  be  to  step  out  of  the  pure  province  of  science,  and  enter 
the  boundless  domain  of  speculation,  since  no  observer  has  ever  seen 
anything  indicating  that  animal  life  exists  on  Mars,  or  on  any  other 
planet  or  satellite.  So  far  as  observation  goes,  Mars  seems  to  be  a 
planet  well  suited  to  sustain  animal  life,  and  we  may  reason  from 
analogy  that  if  animal  life  can  exist  at  all  outside  of  the  Earth,  Mars 
must  have  its  flora  and  fauna  ;  it  must  have  its  fishes  and  birds,  its 
mammalia  and  men  ;  although  all  these  living  beings  must  inevita- 
bly be  very  different  in  appearance  from  their  representatives  on  the 
Earth,  as  can  easily  be  imagined  from  the  differences  existing  be- 
tween the  two  planets.  Although  all  this  is  possible,  and  even  very 
probable,  yet  it  must  be  remembered  that  we  have  not  the  slightest 
evidence  that  it  is  so  ;  and  until  we  have  acquired  this  evidence,  we 
may  only  provisionally  accept  this  idea  as  a  pleasing  hypothesis, 
which,  after  all,  may  be  wrong  and  totally  unfounded. 


ASTRONOMICAL    DRAWINGS.  73 


THE  PLANET  JUPITER 
PLATE  IX. 

JUPITER,  the  giant  of  the  planetary  world,  is  the  fifth  in  order  of 
distance  from  the  Sun,  and  is  next  to  Mars,  our  ruddy  neighbor.  To 
the  naked  eye,  Jupiter  appears  as  a  very  brilliant  star,  whose  mag- 
nitude, changing  with  the  distance  of  this  planet  from  the  Earth, 
sometimes  approaches  that  of  Venus,  our  bright  morning  and  even- 
ing star. 

The  mean  distance  of  Jupiter  from  the  Sun  is  475,000,000  miles, 
but  owing  to  the  eccentricity  of  its  orbit,  its  distance  varies  from  452 
to  498  millions  of  miles.  The  distance  of  this  planet  from  the  Earth 
varies  still  more.  When  nearest  to  our  globe,  or  in  opposition,  its 
distance  is  reduced  to  384,000,000  miles,  and  its  apparent  diameter 
increased  to  50";  while  when  it  is  farthest,  or  in  conjunction,  its  dis- 
tance is  increased  to  567,000,000  miles,  and  its  apparent  diameter 
reduced  to  30";  Jupiter  being  thus  183,000,000  miles  nearer  our  globe 
while  in  opposition  than  when  it  is  in  conjunction. 

This  planet  revolves  around  the  Sun  in  1 1  years,  10  months  and 
17  days,  or  in  only  50  days  less  than  12  terrestrial  years.  Such  is  the 
year  of  this  planet.  The  plane  of  its  orbit  is  inclined  i°  19'  to  the 
ecliptic.  No  planet,  except  Uranus,  has  an  orbit  exhibiting  a 
smaller  inclination.  The  planet  advances  in  its  orbit  at  the  mean 
rate  of  8  miles  a  second;  which  is  a  little  less  than  half  the  orbital 
velocity  of  the  Earth. 

Jupiter  is  of  enormous  proportions.  Its  equatorial  diameter  meas- 
ures 88,000  miles,  and  its  circumference  no  less  than  276,460  miles, 
these  dimensions  being  1 1  times  greater  than  those  of  the  Earth. 
This  planet,  notwithstanding  its  huge  size,  rotates  on  its  axis  in  not 
far  from  9h.  55m.  363.,  which  period  constitutes  its  day.  Owing,  how- 
ever, to  the  changeable  appearance  of  its  surface,  this  period  cannot 
be  ascertained  with  very  great  exactitude.  In  consequence  of  its 
rapid  rotation,  the  planet  is  far  from  spherical,  its  polar  diameter 
being  shorter  than  the  equatorial  by  about  T^,  or  5,500  miles.  Its 


74  THE     TROVVELOT 

surface  is  124  times  the  surface  of  the  earth;  while  its  volume  is 
1,387  times  as  great.  If  Jupiter  occupied  the  place  of  our  satellite 
in  the  sky,  it  would  appear  40  times  as  large  as  the  Moon  appears 
to  us,  and  would  cover  a  surface  of  the  heavens  1,600  times  that 
covered  by  the  full  Moon,  and  would  subtend  an  angle  of  21°. 
Jupiter's  mass  does  not  correspond  with  its  great  bulk,  and  is  only 
Y^j7  of  the  mass  of  the  Sun,  and  310  times  the  mass  of  the  earth; 
its  density  being  only  %  of  that  of  our  globe.  The  force  of  gravita- 
tion at  the  surface  of  this  planet  is  over  2^  times  what  it  is  on  the 
P^arth,  so  that  a  terrestrial  object  carried  to  the  surface  of  Jupiter 
would  weigh  over  two  and  a  half  times  as  much  as  on  our  globe. 

Observed  with  a  telescope,  even  of  moderate  aperture,  Jupiter, 
with  its  four  attending  satellites  and  its  dazzling  brilliancy,  appears 
as  one  of  the  most  magnificent  objects  in  the  sky.  The  general  ap- 
pearance of  the  disk  is  white;  but  unlike  that  of  Mars,  it  is  brightest 
towards  its  central  parts,  and  a  little  darker  around  the  limb,  espe- 
cially on  the  side  opposite  to  the  Sun.  Although  an  exterior  planet, 
and  so  far  from  us,  Jupiter  shows  faint  traces  of  phases  when  ob- 
served near  its  quadratures,  but  this  gibbosity  of  its  disk  is  very 
slight,  and  is  indicated  only  by  a  kind  of  penumbral  shadow  on  the 
limb. 

When  observed  with  adequate  power,  the  disk  of  Jupiter  is  found  to 
be  highly  diversified.  The  principal  features  consist  of  a  series  of  al- 
ternate light  and  dark  streaks  or  bands,  disposed  most  of  the  time 
parallel  with  the  Jovian  equator.  These  bands  differ  from  each 
other  in  intensity  as  well  as  in  breadth;  those  near  the  equator  be- 
ing usually  much  more  prominent  than  those  situated  in  higher  lati- 
tudes north  and  south. 

The  equatorial  zone  of  Jupiter  is  occupied  most  of  the  time  by  a 
broad,  prominent  belt  20°  or  30°  wide,  limited  on  each  side  by  a 
very  dark  narrow  streak.  Between  these  two  dark  borders,  but  sel- 
dom occupying  the  whole  space  between  them,  appears  an  irregular 
white  belt,  apparently  composed  of  dense  masses  of  clouds  strongly 
reflecting  the  Sun's  light,  some  of  these  cloudy  masses  being  very 
brilliant.  The  spaces  left  between  the  cloudy  belt  and  the  dark 
borders,  usually  exhibit  a  delicate  pink  or  rosy  color,  which  pro- 
duces a  very  harmonious  effect  with  the  varying  grayish  and  bluish 
shades  of  some  of  the  belts  and  streaks  seen  on  the  disk.  Quite 
often  the  cloudy  belt  is  broken  up,  and  consists  of  independent 
cloudy  masses,  separated  by  larger  or  smaller  intervals,  these  inter- 
vals disclosing  the  rosy  background  of  this  zone. 

On    each  side  of  the   equatorial   belt  there   is  usually  a  broad 


ASTRONOMICAL     DRAWINGS.  75 

whitish  belt,  succeeded  by  a  narrow  gray  band;  the  space  left  on 
each  hemisphere  between  these  last  bands  and  the  limb  being  usu- 
ally occupied  by  two  or  three  alternate  white  and  gray  bands.  A 
uniform  gray  segment  usually  forms  a  sort  of  polar  cap  to  Jupiter. 

When  observed  under  very  favorable  conditions,  all  the  lighter 
belts  appear  as  if  composed  of  masses  of  small  cloudlets,  resembling 
the  white  opaque  clouds  seen  in  our  atmosphere.  This,  as  already 
stated,  is  particularly  noticeable  on  the  equatorial  belt.  It  is  not 
unusual,  when  Jupiter  is  in  quadrature,  to  see  some  of  the  most  con- 
spicuous white  spots  casting  a  shadow  opposite  to  the  Sun;  a  fact 
which  sufficiently  indicates  that  these  spots  are  at  different  levels. 
They  probably  form  the  summits  of  vast  banks  of  clouds  floating 
high  up  in  the  atmosphere  of  Jupiter. 

What  we  see  of  Jupiter  is  chiefly  a  vaporous,  cloudy  envelope. 
If  our  sight  penetrates  anywhere  deeper  into  the  interior,  it  can  only 
be  through  the  narrow  fissures  of  this  envelope,  which  appear  as 
gray  or  dark  streaks  or  spots.  That  most  of  the  visible  surface  of 
Jupiter  is  simply  a  cloudy  covering,  is  abundantly  proved  by  the 
proper  motion  of  its  spots,  which  sometimes  becomes  very  great. 

In  periods  of  calm,  very  few  changes  are  noticeable  in  the  mark- 
ings of  the  planet,  except,  perhaps,  some  slight  modifications  of  form 
in  the  cloudy,  equatorial  belt  which,  in  general,  is  much  more  liable 
to  changes  than  the  other  belts.  But  the  Jovian  surface  is  not  al- 
ways so  tranquil,  great  changes  being  observed  during  the  terrific 
storms  which  sometimes  occur  on  this  mighty  planet,  when  all  be- 
comes disorder  and  confusion  on  its  usually  calm  surface;  and  noth- 
ing on  the  Earth  can  give  us  a  conception  of  the  velocity  with 
which  some  of  its  clouds  and  spots  are  animated.  New  belts  quickly 
form,  while  old  ones  disappear.  The  usual  parallelism  of  the  belts 
no  more  exists.  Huge,  white,  cumulus-like  masses  advance  and 
spread  out,  the  rosy  equatorial  belt  enlarges  sometimes  to  two  or 
three  times  its  usual  size,  and  occupies  two-thirds,  or  more,  of  the 
disk,  the  rosy  tint  spreading  out  in  a  very  short  time.  At  times 
very  dark  bands  extending  across  the  disk  are  transformed  into 
knots  or  dark  spots,  which  encircle  the  planet  with  a  belt,  as  it 
were,  of  jet  black  beads.  Sometimes,  also,  a  secondary  but  nar- 
rower rosy  belt  forms  either  in  the  northern  or  the  southern  hemi- 
sphere, and  remains  visible  for  a  few  days  or  for  years  at  a  time. 

On  May  25,  1876,  I  witnessed  one  of  the  grandest  commotions 
which  can  be  conceived  as  taking  place  in  an  atmosphere.  All  the 
southern  hemisphere  of  Jupiter,  from  equator  to  pole,  was  in  rapid 
motion,  the  belts  and  spots  being  transported  entirely  across  the 


76  THE    TROUVELOT 

disk,  from  the  eastern  to  the  western  limb,  in  one  hour's  time,  during- 
which  the  equatorial  belt  swelled  to  twice  its  original  breadth, 
towards  the  south. 

Now,  when  one  stops  for  a  moment  to  think  what  is  signified  by 
that  motion  of  the  dark  spots  across  the  little  telescopic  disk  of  Jupi- 
ter in  an  hour's  time,  he  may  arrive  at  some  conception  of  the  mag- 
nitude of  the  Jovian  storms,  compared  with  those  of  our  globe.  The 
circumference  of  Jupiter's  equator,  as  stated  above,  is  276,460  miles; 
half  this  number,  or  138,230  miles,  represents  the  length  of  the 
equatorial  line  seen  from  the  Earth.  Now,  after  taking  into  account 
the  rotation  of  the  planet,  which  somewhat  diminishes  the  apparent 
motion,  we  arrive  at  the  astonishing  result  that  the  spots  and  mark- 
ings were  carried  along  by  this  Jovian  storm,  at  the  enormous  rate 
of  1 10,584  miles  an  hour,  or  over  30.7  miles  a  second.  On  our  globe,  a 
hurricane  or  tornado,  which  blows  at  the  rate  of  100  miles  an  hour, 
sweeps  everything  before  it.  What,  then,  must  be  expected  from  a 
velocity  over  1,105  times  as  great  ?  Enormous  as  this  motion  may 
appear,  its  occurrence  cannot  be  doubted,  since  it  is  disclosed  by 
direct  observation. 

The  surface  of  Jupiter,  it  would  seem,  has  its  periods  of  calm  and 
activity  like  that  of  the  Sun,  although  it  is  not  yet  known,  as  it  is 
for  the  latter,  that  they  recur  with  approximate  regularity. 

My  observations  of  this  planet,  which  embrace  a  period  of  ten 
years,  seem  to  point  in  that  direction,  for  they  show,  at  least,  that 
Jupiter  has  its  years  of  calm  and  its  years  of  disturbances.  The  year 
1876  was  a  year  of  extraordinary  disturbance  on  Jupiter.  Changes 
in  the  markings  were  going  on  all  the  time,  and  no  one  form  could 
be  recognized  the  next  day,  or  even  sometimes  the  next  hour,  as 
shown  above.  The  cloudy  envelope  of  the  planet  was  in  constant 
motion,  the  equatorial  belt,  especially,  showing  the  signs  of  greatest 
disturbance,  being,  for  the  most  part,  two  or  three  times  as  wide  as 
in  other  years.  After  1876  the  calm  was  very  great  on  the  planet, 
only  a  slight  change  now  and  then  being  noticeable,  the  same  forms 
being  recognized  day  after  day,  month  after  month,  and  even  year 
after  year.  In  one  case  the  same  marking  has  been  observed  for 
seventeen  consecutive  months,  and  in  another  for  twenty-eight 
months.  This  state  of  quietude  lasted  until  October,  1880,  when 
considerable  commotion  occurred  on  the  northern  hemisphere,  where 
large  round  black  spots,  somewhat  resembling  the  Sun-spots,  formed 
in  the  cloudy  atmosphere,  and  finally  changed,  towards  the  end  of 
December,  into  a  narrow  pink  belt,  which  still  exists. 

The  most  curious  marking  ever  seen  on  Jupiter  is  undoubtedly 


ASTRONOMICAL     DRAWINGS.  77 

the  great  Red  Spot,  observed  on  the  southern  hemisphere  of  this 
planet  for  the  last  three  years.  This  interesting  object,  seen  first  in 
July,  1878,  disappeared  for  a  time,  reappeared  on  September  25  of  the 
same  year,  and  has  remained  visible  until  now.  When  seen  by  me 
in  September,  it  was  much  elongated,  and  sharply  pointed  on  one 
side,  like  a  spear-head,  but  it  subsequently  acquired  an  irregular  form, 
with  short  appendages  protruding  from  its  northern  border.  At 
first,  the  changes  were  great  and  frequent,  but  at  length  it  acquired 
the  regular  oval  form,  which,  with  but  slight  modifications,  it  has 
retained  until  now.  During  the  month  of  November,  1880,  I  noticed 
two  small  black  specks  upon  this  Red  Spot,  and  they  were  seen  again 
in  January  of  the  succeeding  year,  by  Mr.  Alvan  Clark,  Jr.  When  the 
spot  had  attained  its  oval  shape,  it  appeared  part  of  the  time  sur- 
rounded with  a  white  luminous  ring  of  cloudy  forms  which,  however, 
was  changing  more  or  less  all  the  time,  being  sometimes  invisible. 
The  color  of  this  curious  spot  is  a  brilliant  rosy  red,  tinged  with  ver- 
milion, and  altogether  different  in  shade  from  the  pinkish  color  of 
the  equatorial  belt.  The  size  of  the  spot  varies,  but  of  late  its 
changes  have  been  slight.  Its  longer  diameter  may  be  estimated  at 
8,000  miles,  and  the  shorter  at  2,200  miles.  The  Red  Spot  is  repre- 
sented on  Plate  IX.  with  its  natural  color,  and  as  it  appears  at  the 
moments  most  favorable  for  observation.  In  ordinary  cases  its  color 
does  not  appear  so  brilliant,  but  paler. 

It  is  difficult  to  account  for  the  color  of  the  equatorial  belt  and 
that  of  the  Red  Spot;  but  it  is  known,  at  least,  that  the  material  to 
which  they  are  due  cannot  be  situated  at  the  level  of  the  general 
surface  visible  to  us,  and  especially  that  of  the  cloudy  forms  of  the 
equatorial  zone.  Undoubtedly  the  red  layer  lies  deeper  than  the 
superficial  envelope  of  the  planet,  although  it  does  not  seem  to  be 
very  deeply  depressed. 

Jupiter  is  attended  by  four  satellites,  which  revolve  around  the 
planet  at  various  distances,  and  shine  like  stars  of  the  6th  and  /th 
magnitude.  It  is  said  that  under  very  favorable  circumstances,  and 
in  a  very  clear  sky,  the  satellites  can  be  seen  with  the  naked  eye, 
but  this  requires  exceptionally  keen  eyes,  since  the  glare  of  the 
planet  is  so  strong  as  to  overpower  the  comparatively  faint  light  of 
the  satellites.  However,  I  myself  have  sometimes  seen,  without  the 
aid  of  the  telescope,  two  or  three  of  the  satellites '  as  a  single 
object,  when  they  were  closely  grouped  on  the  same  side  of  Jupiter. 

The  four  moons  of  Jupiter  are  ail  larger  than  our  Moon,  except 
the  first,  which  has  about  the  same  diameter.  They  range  in  size 
from  2,300  to  3,400  miles  in  diameter,  the  third  being  the  largest ; 


78  THE    TROUVELOT 

the  determination  of  their  diameter  is  by  no  means  accurate,  how- 
ever, as  it  is  difficult  to  measure  such  small  objects  with  precision. 
Their  mean  distance  from  the  centre  of  Jupiter  varies  from  267,000 
to  1,192,000  miles,  the  first  satellite,  the  nearest  to  the  planet, 
being  a  little  farther  from  Jupiter  then  our  satellite  is  from  us.  The 
four  satellites  revolve  around  the  planet  in  orbits  whose  planes  have 
a  slight  inclination  to  the  equator  of  Jupiter,  and  consequently  to  the 
ecliptic.  The  diameter  of  the  largest  satellite  is  nearly  half  that  of 
the  Earth,  or  3,436  miles;  while  its  volume  is  five  times  that  of  our 
Moon.  The  period  of  revolution  of  these  satellites  varies  from  id. 
i8h.  for  the  first,  to  i6d.  i6h.  for  the  last. 

Owing  to  the  slight  inclination  of  the  pla,ne  of  their  orbits  to  that 
of  the  planet,  the  three  first  satellites,  and  generally  the  fourth, 
pass  in  front  of  the  disk  and  also  through  the  shadow  of  the  planet 
at  every  revolution,  and  are  accordingly  eclipsed.  Their  passages 
behind  Jupiter's  disk  are  called  occultations;  those  in  front  of  it, 
transits.  The  eclipses,  the  occultations  and  the  transits  of  the  moons 
of  Jupiter  are  interesting  and  important  phenomena  ;  the  eclipses 
being  sometimes  observed  for  the  rough  determination  of  longitudes 
at  sea. 

The  satellites  in  transit  present  some  curious  phenomena.  When 
they  enter  the  disk,  they  appear  intensely  luminous  upon  its  gray- 
ish border  ;  but  as  they  advance,  they  seem  by  degrees  to  lose 
their  brightness,  until  they  finally  become  undistinguishable  from 
the  luminous  surface  of  Jupiter.  It  sometimes  happens,  however, 
that  the  first,  the  third  and  the  fourth  satellites,  after  ceasing  to 
appear  as  bright  spots,  continue  to  be  visible  as  dark  spots  upon 
the  bright  central  portions  of  the  planet's  disk  ;  but  in  these  cases 
their  disks  appear  smaller  than  the  shadows  they  cast.  Undoubt- 
edly these  satellites  have  extensive  atmospheres,  since  they  some- 
times pass  unperceived  across  the  central  parts  of  Jupiter,  this  being 
probably  when  their  atmospheres  are  condensed  into  clouds,  strong- 
ly reflecting  light  ;  while  when  these  clouds  are  absent,  we  can  see 
their  actual  surface,  with  traces  of  the  dark  spots  upon  them  similar 
to  those  on  Mars. 

From  the  variation  in  the  brightness  of  these  satellites,  which  is 
said  to  be  always  observed  in  the  same  part  of  their  orbit,  William 
Herschel  was"  led  to  suppose  that  these  bodies,  like  our  Moon, 
rotate  upon  their  axes  in  the  same  period  in  which  they  move  round 
the  planet,  so  that  they  always  present  the  same  face  to  Jupiter  ; 
but  these  conclusions  have  been  denied.  From  my  observations 
it  is  apparent,  however,  that  the  light  reflected  by  them  varies  in 


ASTRONOMICAL     DRAWINGS.  79 

intensity  as  well  as  in  color.  But  this  is  rather  to  be  attributed  to 
the  presence  of  an  atmosphere  surrounding  these  bodies,  which 
when  cloudy  reflects  more  light  than  when  clear,  with  correspond- 
ing changes  in  the  color  of  the  light. 

The  satellites  in  transit  are  sometimes  preceded  or  followed, 
according  to  the  position  of  the  Sun,  by  a  round  black  spot  having 
about  the  same  size  as  the  satellite  itself.  This  black  spot  is  the 
shadow  of  the  satellite  cast  on  the  vapory  envelope  of  Jupiter, 
similar  to  the  shadow  cast  by  the  Moon  on  the  Earth,  during 
eclipses  of  the  Sun  ;  in  fact,  all  the  Jovian  regions  traversed  by 
these  shadows  have  the  Sun  totally  eclipsed.  Sometimes  it  happens 
that  the  shadow  appears  elliptical.  This  occurs  either  when  it  is 
observed  very  near  the  limb,  or  when  entering  upon  a  round,  cloud- 
like  spot.  This  effect  is  attributable  to  the  perspective  under  which 
the  shadow  is  seen  on  the  spherical  globe  or  spot. 

The  proper  motion  of  the  satellites  in  the  Jovian  sky  is  much 
more  rapid  than  that  of  the  Moon  in  our  sky.  During  one  Jovian 
day  of  ten  hours,  the  first  satellite  advances  84°  ;  the  second,  42°  ; 
the  third,  20°  and  the  fourth,  9°.  The  first  satellite  passes  from 
New  Moon  to  its  first  quarter  in  a  little  more  than  a  Jovian  day, 
while  the  fourth  occupies  ten  such  days  in  attaining  the  same  phase. 

In  density,  as  well  as  in  physical  constitution,  Jupiter  differs 
widely  from  the  interior  planets,  and  especially  from  the  Earth;  and, 
as  has  been  shown,  it  is  surrounded  by  a  dense,  opaque,  cloudy  layer, 
which  is  almost  always  impenetrable  to  the  sight,  and  hides  from 
view  the  nucleus,  which  we  may  conceive  to  exist  under  this  va- 
porous envelope.  In  1876,  the  year  of  the  great  Jovian  disturbances, 
I  observed  frequently  in  the  northern  hemisphere  of  the  planet  a 
very  curious  phenomenon,  which  seems  to  prove  that  its  cloudy  en- 
velope is  at  times  partially  absent  in  some  places,  its  vapors  being 
apparently  either  condensed,  or  transported  to  other  parts  of  its  sur- 
face, and  that,  therefore,  a  considerable  part  of  the  real  globe  of  the 
planet  was  visible  at  these  places.  The  phenomenon  consisted  in 
the  deformation  of  the  northern  limb,  which  had  a  much  shorter  ra- 
dius on  all  of  this  hemisphere  situated  northward  of  the  white  belt 
which  adjoins  the  equatorial  zone.  The  deformation  of  the  limb 
on  both  sides,  where  it  passed  from  a  longer  to  a  shorter  radius, 
was  abrupt,  and  at  right  angles  to  the  limb,  forming  there  a  step- 
like  indentation  which  was  very  prominent.  The  polar  segment 
having  a  smaller  radius,  appeared  unusually  dark,  and  was  not 
striped,  as  usual,  but  uniform  in  tint  throughout.  On  September 
27th,  the  third  satellite  passed  over  this  dark  segment,  and  emerged 


80  THE    TROUVELOT 

from  the  western  border,  a  little  below  the  place  where  the  limb 
was  abruptly  deformed,  as  above  described.  When  the  satellite  had 
fully  emerged  from  this  limb,  it  was  apparent  that  if  the  portion  of 
the  limb  having  a  longer  radius  had  been  prolonged  a  little  below, 
and  as  far  as  the  satellite,  it  would  have  enclosed  it  within  its  bor- 
der, and  thus  retarded  the  time  of  emersion.  The  depth  of  defor- 
mation of  the  limb  was  accordingly  greater  than  the  diameter  of  the 
third  satellite,  and  certainly  more  than  4,000  miles.  That  the  phe- 
nomenon was  real,  is  proved  by  the  fact  that  the  egress  of  this  sat- 
ellite occurred  at  least  four  minutes  sooner  than  the  time  predicted 
for  it  in  the  American  Ephemeris.  Other  observations  seem  to 
point  in  the  same  direction,  since  some  of  the  satellites  which  were 
occulted  have  been  seen  through  the  limb  of  Jupiter  by  different  as- 
tronomers, as  if  this  limb  was  sometimes  semi-transparent.  Another 
observation  of  mine  seems  to  confirm  these  conclusions.  On  April 
24th,  1877,  at  I5h.  25m.  the  shadow  of  the  first  satellite  was  pro- 
jected on  the  dark  band  forming  the  northern  border  of  the  equa- 
torial belt,  the  shadow  being  then  not  far  from  the  east  limb.  Close 
to  this  shadow,  and  on  its  western  side,  it  was  preceded  by  a  sec- 
ondary shadow,  which  was  fainter,  but  had  the  same  apparent  size. 
This  round  dark  spot  was  not  the  satellite  itself,  as  I  had  supposed 
at  first,  since  this  object  was  yet  outside  of  the  planet,  on  the  east, 
and  entered  upon  it  only  at  i6h.  4m.  I  watched  closely  this  strange 
phenomenon,  and  at  i6h.  45m.,  when  the  shadow  had  already  crossed 
about  ^  of  the  disk,  it  was  still  preceded  by  the  secondary,  or  mock 
shadow,  as  it  may  be  called;  the  same  relative  distance  having  been 
kept  all  the  while  between  the  two  objects,  which  had  therefore 
traveled  at  the  same  rate.  It  is  obvious  that  this  dark  spot  could 
not  be  one  of  the  planet's  markings,  since  the  shadow  of  the  first 
satellite  moves  more  quickly  on  the  surface  of  Jupiter  than  a  spot  on 
the  same  surface  travels  by  the  effect  of  rotation,  so  that  in  this  case 
the  shadow  would  soon  have  passed  over  this  marking,  and  left  it 
behind,  during  the  time  occupied  by  the  observation.  From  these 
observations  it  seems  very  probable  that  Jupiter  has  a  nucleus,  either 
solid  or  liquid,  which  lies  several  thousand  miles  below  the  surface 
of  its  cloudy  envelope.  It  is  also  probable  that  the  uniformly  shaded 
dark  segment  seen  in  1876,  was  a  portion  of  the  surface  of  this  nu- 
cleus itself.  When  the  cloudy  envelope  is  semi-transparent  at  the 
place  situated  on  a  line  with  an  occulted  satellite  and  the  eye  of  an 
observer,  this  satellite  may  accordingly  remain  visible  for  a  time 
through  the  limb,  as  shown  by  observation.  The  phenomenon  of 
the  mock  shadow  may  also  be  attributed  to  a  similar  cause,  where 


ASTRONOMICAL     DRA  WINGS.  81 

semi-transparent  vapors  receive  the  shadow  of  a  satellite  at  their 
surface,  while  at  the  same  time  part  of  this  shadow,  passing  through 
the  semi-transparent  vapors,  may  be  seen  at  the  surface  of  the  nu- 
cleus, or  of  a  layer  of  opaque  clouds  situated  at  some  distance  below 
the  surface. 

Some  astronomers  are  inclined  to  think  that  Jupiter  is  at  a  high 
temperature,  and  self-luminous  to  a  certain  extent.  If  this  planet  is 
self-luminous  to  any  degree,  we  might  expect  that  some  light  would 
be  thrown  upon  the  satellites  when  they  are  crossing  the  shadow 
cast  into  space  by  the  planet;  but  when  they  cross  this  shadow  they 
are  totally  invisible  in  the  best  telescopes,  a  proof  that  they  do  not 
receive  much  light  from  the  non-illuminated  side  of  Jupiter.  It 
would,  indeed,  seem  probable  that  some  of  the  intensely  white  spots 
occasionally  seen  on  the  equatorial  belt  of  the  planet  are  self-lumin- 
ous in  a  degree,  yet  not  enough  to  render  the  satellites  visible  while 
they  are  immersed  in  Jupiter's  shadow.  It  does  not  seem  impossible 
that  the  planet  should  have  the  high  temperature  attributed  to  it, 
when  we  remember  the  terrific  storms  observed  in  its  atmosphere, 
which,  owing  to  the  great  distance  of  Jupiter  from  the  Sun,  do  not 
seem  to  be  attributable  to  this  body,  but  rather  to  some  local  cause 
within  the  envelope  of  the  planet. 

Astronomy,  which  is  a  science  of  observation,  is  naturally  silent 
with  regard  to  the  inhabitants  of  Jupiter.  If  there  are  any  such  in- 
habitants, they  are  confined  to  the  domain  of  conjecture,  under  the 
dense  cloudy  envelope  of  the  planet.  The  conditions  of  habitability 
on  Jupiter  must  differ  very  widely  from  those  of  our  globe.  Com- 
paratively little  direct  light  from  the  Sun  reaches  the  surface  of  the 
globe  of  Jupiter,  except  that  which  passes  through  the  narrow 
openings  forming  the  dark  clouds.  All  the  rest  of  the  planet's  sur- 
face, being  covered  perpetually  by  opaque  clouds,  receives  only  dif- 
fused light.  On  Jupiter  there  are  practically  no  seasons,  since  its 
axis  is  nearly  perpendicular  to  its  orbit.  The  force  of  gravity  on  the 
surface  of  Jupiter  being  more  than  double  what  it  is  on  the  Earth, 
living  bodies  would  there  have  more  than  double  the  weight  of  sim- 
ilar bodies  on  the  Earth.  Furthermore,  Jupiter  only  receives  o.oii 
of  the  light  and  heat  which  we  receive  from  the  Sun  ;  and  its 
year  is  nearly  equal  to  12  of  our  years.  If  there  are  living  beings  on 
Jupiter,  they  must,  then,  be  entirely  different  from  any  known  to  us, 
and  they  may  have  forms  never  dreamed  of  in  our  most  fantastic 
conceptions. 

The  two  round  black  spots  represented  towards  the  central  parts 
of  Plate  IX.  are  the  shadows  of  the  first  and  second  satellite  ;  while 


82 


THE     TROUVELOT 


the  two  round  white  spots  seen  on  the  left  of  the  disk,  are  the  satel- 
lites themselves,  as  they  appeared  at  the  moment  of  the  observa- 
tion. The  first  satellite  and  its  shadow  are  the  nearest  to  the 
equator  ;  while  the  second  satellite  and  its  shadow  are  higher,  the 
last  being  projected  on  the  Great  Red  Spot.*  The  row  of  dark  cir- 
cular spots  represented  on  the  northern,  or  lower  hemisphere,  when 
they  first  appeared,  had  some  resemblance  to  Sun-spots  without 
a  penumbra,  with  bright  markings  around  them,  resembling  faculae. 
These  round  spots  subsequently  enlarged  considerably,  until  they 
united  along  the  entire  line,  encircling  the  planet,  and  finally  form- 
ing a  narrow  pink  belt,  which  is  still  visible. 

*  Note. — By  an  accidental  error  in  enlarging  the  original  drawing,  the  satellites  and 
shadows  appear  in  Plate  IX.  of  double  their  actual  size.  The  error  is  one  easy  of  mental 
correction. 


A  S TRONOMICA  L     DRA  WINGS.  83 


THE  PLANET  SATUKN. 
PLATE  X. 

SATURN,  which  is  next  to  Jupiter  in  order  of  distance  from  the 
Sun,  while  not  the  largest,  is  certainly  the  most  beautiful  and  inter- 
esting of  all  the  planets,  with  his  grand  and  unique  system  of  rings, 
and  his  eight  satellites,  which,  like  faithful  servants,  attend  the 
planet's  interminable  journey  through  space. 

Seen  with  the  naked  eye,  Saturn  shines  in  the  night  like  a  star  of 
the  first  magnitude,  whose  dull,  soft  whiteness  is,  however,  far  from 
attaining  the  brilliancy  of  Venus  or  Jupiter,  although  it  sometimes 
approaches  Mars  in  brightness.  Saturn  hardly  ever  exhibits  the 
phenomenon  of  scintillation,  or  twinkling,  a  peculiarity  which 
makes  it  easily  distinguishable  among  the  stars  and  planets  of  the 
heavens. 

The  synodical  period  of  Saturn  occupies  I  year  and  13  days,  so 
that  every  378  days,  on  an  average,  this  planet  holds  the  same  posi- 
tion in  the  sky  relatively  to  the  Sun  and  the  Earth. 

The  mean  distance  of  Saturn  from  the  Sun  is  a  little  over 
9%  times  that  of  our  globe,  or  872,000,000  miles.  Owing  to  the  or- 
bital eccentricity,  this  distance  may  increase  to  921,000,000  miles, 
when  the  planet  is  in  aphelion  ;  or  decrease  to  823,000,000  miles, 
when  it  is  in  perihelion  ;  Saturn  being  therefore  98,000,000  miles 
nearer  to  the  Sun  when  in  perihelion  than  in  aphelion.  If  gravita- 
tion were  free  to  exert  its  influence  alone,  Saturn  would  fall  into 
the  Sun  in  5  years  and  2  months. 

The  distance  of  Saturn  from  the  Earth  varies,  according  to  the 
position  of  the  two  planets  in  their  respective  orbits.  At  the  time 
of  opposition,  when  the  Earth  lies  between  the  Sun  and  Saturn,  this 
distance  is  smallest  ;  while,  on  the  contrary,  at  the  time  of  conjunc- 
tion, when  the  Sun  lies  between  the  Earth  and  Saturn,  it  is  great- 
est. Owing,  however,  to  the  eccentricity  of  the  orbits  of  Saturn  and 
our  globe,  and  the  inclination  of  their  planes  to  each  other,  and 
owing  also  to  the  variable  heliocentric  longitude  of  the  perihelion, 


84  THE    TROUVELOT 

the  distance  of  the  two  planets  from  each  other  at  their  successive 
conjunctions  and  oppositions  is  rendered  extremely  variable.  At 
present  it  is  when  the  oppositions  of  Saturn  occur  in  December  that 
this  planet  comes  nearest  to  us  ;  while  when  the  conjunctions  take 
place  in  June,  the  distance  of  Saturn  from  the  Earth  is  the  greatest 
possible.  In  the  former  case  the  distance  of  the  planet  from  our 
globe  is  only  730,000,000  miles  ;  while  in  the  last  it  is  1,014,000,000 
miles,  the  difference  between  the  nearest  and  farthest  points  of 
Saturn's  approach  to  us  being  no  less  than  284,000,000  miles,  or  over 
three  times  the  mean  distance  of  the  Sun  from  the  Earth. 

From  the  great  variations  in  the  distance  of  Saturn  from  the  Earth, 
necessarily  result  corresponding  changes  in  the  brightness  and  ap- 
parent diameter  of  this  body.  When  it  is  farthest  from  us,  its  angu- 
lar diameter  measures  but  14"  ;  while,  when  it  is  nearest,  it  meas- 
ures 20". 

The  orbit  of  Saturn  is  inclined  2°3O'  to  the  ecliptic,  and  its  eccen- 
tricity, which  equals  0,056,  is  over  three  times  that  of  the  Earth's 
orbit. 

This  planet  revolves  around  the  Sun  in  a  period  of  29  years  and 
5^4  months,  or  10,759  terrestrial  days,  which  constitutes  its  sidereal 
year.  The  extension  of  the  immense  curve  forming  the  .orbit  of  this 
planet,  is  no  less  than  5,505,000,000  miles,  which  is  traversed  by 
the  planet  with  a  mean  velocity  of  a  little  less  than  6  miles  per 
second,  or  three  times  less  than  the  motion  of  our  globe  in  space. 

The  real  dimensions  of  the  globe  of  Saturn  are  not  yet  known 
with  accuracy,  and  the  equatorial  diameter  has  been  variously  esti- 
mated by  observers,  at  from  71,000  to  79,000  miles.  If  we  adopt  the 
mean  of  these  numbers,  75,000  miles,  the  circumference  of  the 
Saturnian  equator  would  measure  235,620  miles,  or  g%  times  the 
circumference  of  our  globe  ;  the  surface  of  Saturn  would  be  86  times, 
and  its  volume  over  810  times  that  of  the  Earth. 

However  great  the  volume  of  Saturn,  its  mass  is  proportionally 
small,  being  only  90  times  greater  than  that  of  our  globe  ;  the  mean 
density  of  the  materials  composing  this  planet  being  less  than  that 
of  cork,  and  only  0.68  the  density  of  water.  The  force  of  gravitation 
at  the  surface  of  Saturn  is  greater,  by  a  little  over  -£-,  than  it  is  at 
the  surface  of  the  Earth  ;  a  body  falling  in  a  vacuum  at  its  surface, 
would  travel  17.59  feet  during  the  first  second. 

From  observations  of  markings  seen  on  the  surface  of  Saturn,  and 
from  the  study  of  their  apparent  displacements  on  the  disk,  William 
Herschel  found  that  the  planet  rotated  upon  its  axis  in  loh.  i6m. 
0.245.  Since  Herschel's  determination,  new  researches  have  been 


ASTRONOMICAL     DRA  WINGS.  85 

made,  and  lately,  Professor  Hall,  noticing  a  bright  spot,  followed  it 
for  nearly  a  month,  observing  its  transits  across  the  central  meri- 
dian of  the  disk.  From  these  observations  he  has  obtained  for  the 
rotation  period  loh.  14111.  23.83.,  a  result  which  agrees  very  closely  with 
that  obtained  82  years  earlier  by  Herschel,  considering  the  fact  that 
the  markings  from  which  the  period  of  rotation  is  ascertained  are 
not  fixed  on  the  planet,  but  are  always  more  or  less  endowed  with 
proper  motion.  The  velocity  of  rotation  at  the  equator  is  21,538 
miles  per  hour,  or  nearly  6  miles  per  second. 

The  axis  of  rotation  of  Saturn  is  inclined  64°  18'  to  the  plane  of 
the  orbit,  so  that  its  equator  makes  an  angle  of  25°  42'  with  the 
same  plane.  The  seasons  of  this  planet  therefore  present  greater 
extremes  of  temperature  than  those  of  the  Earth,  but  not  quite  so 
great  variations  as  the  seasons  of  Mars. 

The  globe  of  Saturn  is  not  a  perfect  sphere,  but  its  figure  is  that 
of  an  oblong  spheroid,  flattened  at  the  poles.  The  polar  compression 
of  Saturn  is  greater  than  that  of  any  other  planet,  surpassing  even 
that  of  Jupiter.  Though  not  yet  determined  with  a  great  degree  of 
accuracy,  the  compression  is  known  to  be  between  \  and  -^  of  the 
equatorial  diameter  ;  that  is,  a  flattening  of  about  3, 894  miles,  at  each 
pole,  the  polar  diameter  being  7,788  miles  shorter  than  the  equa- 
torial. 

The  internal  condition  of  the  planet  Saturn,  whether  solid,  liquid 
or  gaseous,  cannot  be  discovered  from  the  examination  of  its  surface, 
as  its  globe  is  enwrapped  in  a  dense  opaque  layer  of  vapors  and 
cloud-like  forms,  through  which  the  sight  fails  to  penetrate.  The 
appearance  of  this  vapory  envelope  is  like  that  of  cumulus  clouds, 
and  one  of  its  characteristics  is  to  arrange  itself  into  alternate  bright 
and  dark  parallel  belts,  broader  than  those  seen  on  Jupiter,  and  also 
more  regular  and  dark.  These  belts,  which  are  parallel  to  the 
equator  of  the  planet,  vary  in  curvature  with  the  inclination  of  its 
axis  of  rotation  to  the  line  of  sight. 

The  belts  of  Saturn,  like  those  of  Jupiter,  are  not  permanent,  but 
keep  changing  more  or  less  rapidly.  Sometimes  they  have  been 
observed  to  be  quite  numerous  ;  while  at  other  times  they  are  few. 
Occasionally  conspicuous  white  or  dark  spots  are  seen  on  the 
surface,  although  the  phenomenon  is  quite  rare.  It  is  from  the 
observation  of  such  spots  that  Saturn's  period  of  rotation  has  been 
determined,  as  stated  above.  The  equatorial  zone  of  Saturn  always 
appears  more  white  and  brilliant  than  the  other  parts,  as  it  also  ap- 
pears more  mottled  and  cloud-like.  In  late  years  the  globe  has 
been  characterized,  and  much  adorned,  by  a  pale  pinkish  tint  on 


86  THE    TROUVELOT 

its  equatorial  belt,  resembling  that  of  Jupiter,  but  somewhat  fainter. 
On  either  side  of  the  equatorial  belt  there  is  a  narrower  bandr 
upon  which  the  mottled  appearance  is  visible.  Below  these,  one 
or  two  dark  belts,  separated  by  narrow  white  bands,  are  usually 
seen  ;  but,  of  late,  the  bands  have  been  less  numerous,  being  re- 
placed in  high  latitudes  by  a  dark  segment,  which  forms  a  polar 
cap  to  Saturn.  The  globe  of  Saturn  does  not  anywhere  appear 
perfectly  white,  and  when  compared  with  its  ring,  it  looks  of  a 
smoky  yellowish  tint,  which  becomes  an  ashy  gray  on  its  shaded 
parts.  It  usually  appears  darker  near  the  limb  than  in  its  central 
portions  ;  although  on  some  occasions  I  have  seen  portions  of  the 
limb  appear  brighter,  as  if  some  white  spots  were  traversing  it. 

Some  observers  have  seen  the  limb  deformed  and  flattened  at 
different  places,  and  W.  Herschel  even  thought  such  a  deformation 
to  be  a  permanent  feature  of  this  globe,  which  he  termed  diamond- 
shaped,  or  "  square  shouldered."  But  this  was  evidently  an  illusion, 
since  the  planet's  limb  usually  appears  perfectly  elliptical,  although 
it  occasionally  appears  as  if  flattened  at  some  points,  especially  where 
it  comes  in  apparent  contact  with  the  shadow  cast  by  the  globe  on 
the  ring,  as  observed  by  myself  many  times.  But  with  some  atten- 
tion, it  is  generally  found  that  this  deformation  is  apparent  rather 
than  real,  and  is  caused  by  the  passage  of  some  large  dark  spots 
over  the  limb,  which  is  thus  rendered  indistinguishable  from  the  dark 
background  upon  which  it  is  projected. 

What  distinguishes  Saturn  from  all  known  planets,  or  heavenly 
bodies,  and  makes  it  unique  in  our  universe,  is  the  marvelous  broad 
flat  ring  which  encircles  its  equator  at  a  considerable  distance  from 
it.  With  a  low  magnifying  power  this  flat  ring  appears  single,  but 
when  carefully  examined  with  higher  powers,  it  is  found  to  consist  of 
several  distinct  concentric  rings  and  zones,  all  lying  nearly  in  the 
same  plane  with  the  planet's  equator. 

At  first  sight  only  two  concentric  rings  are  recognized,  the  outer 
and  the  middle,  or  intermediary,  which  are  separated  by  a  wide  and 
continuous  black  line,  called  the  principal  division.  This  line,  and 
indeed  all  the  features  of  the  surface  of  the  rings  are  better  seen,  and 
appear  more  prominent  on  that  part  of  the  ring  on  either  side  called 
the  ansa,  or  handle.  Besides  these  two  conspicuous  rings,  a  third, 
of  very  dark  bluish  or  purplish  color,  lies  between  this  middle  ring, 
to  which  it  is  contiguous,  and  the  planet.  This  inner  ring,  which  is 
quite  wide,  is  called  the  gauze  or  dusky  ring.  Closer  examination 
shows  that  the  outer  ring  is  itself  divided  by  a  narrow,  faint,  grayish 
line  called  the  pencil  line,  which,  from  its  extreme  faintness,  is  only 


ASTRONOMICAL     DRAWINGS.  87 

visible  on  the  ansae.  Moreover,  the  middle  ring  is  composed  of  three 
concentric  zones,  or  belts,  which,  although  not*  apparently  divided 
by  any  interval  of  space,  are  distinguished  by  the  different  shadings 
of  the  materials  composing  them.  The  outer  zone  of  this  compound 
middle  ring  is,  by  far,  the  brightest  of  all  the  system  of  rings  and 
belts,  especially  close  to  its  external  border,  where,  on  favorable 
occasions,  I  have  seen  it  appear  on  the  ansae  as  if  mottled  over,  and 
covered  throughout  with  strongly  luminous  cloud-like  masses.  On 
the  ansae  of  the  double  outer  ring,  similar  cloudy  forms  have  also 
been  seen  at  different  times.  The  second  zone  of  the  middle  ring  is 
darker  than  the  first,  the  innermost  being  darker  still.  All  the 
characteristic  points  which  have  thus  been  described,  are  shown  in 
Plate  X. 

Although  suspected  in  1838,  the  dusky  ring  was  not  recognized 
before  1850,  when  G.  P.  Bond  discovered  it  with  the  1 5-inch  refrac- 
tor of  the  Cambridge  Observatory.  It  was  also  independently  dis- 
covered the  same  year  in  England  by  Dawes  and  Lassell.  The 
dusky  ring  differs  widely  in  appearance  and  in  constitution  from  the 
other  rings,  inasmuch  as  these  last  are  opaque,  and  either  white  or 
grayish,  while  the  former  is  very  dark,  and  yet  so  transparent  that  the 
.  limb  of  the  planet  is  plainly  seen  through  its  substance.  On  particu- 
larly favorable  occasions,  the  appearance  of  this  ring  resembles  that 
of  the  fine  particles  of  dust  floating  in  a  ray  of  light  traversing  a  dark 
chamber.  Whatever  may  be  the  material  of  which  this  ring  is  com- 
posed, it  must  be  quite  rarefied,  especially  towards  its  inner  border, 
which  appears  as  if  composed  of  distinct  and  minute  particles  of  mat- 
ter feebly  reflecting  the  solar  light.  That  the  inner  part  of  the  dusky 
ring  is  composed  of  separate  particles,  is  proved  by  the  fact  that  the 
part  of  the  ring  which  is  seen  in  front  of  the  globe  of  Saturn  has  its 
inner  border  abruptly  deflected  and  curved  inward  on  entering  upon 
the  disk,  causing  it  to  appear  considerably  narrower  than  it  must  be 
in  reality,  a  peculiarity  which  is  shown  in  the  Plate.  This  phe- 
nomenon may  be  attributed  to  an  effect  of  irradiation,  due  to  the 
strong  light  reflected  by  the  central  parts  of  the  ball,  which  so  re- 
duces the  apparent  diameter  of  the  individual  particles  that  they 
become  invisible  to  us,  especially  those  near  the  inner  border,  which 
are  more  scattered  and  less  numerous  than  elsewhere. 

The  dusky  ring,  which  was  described  by  Bond,  Lassell  and  other 
astronomers  as  being  equally  transparent  throughout  all  its  width, 
has  not  been  found  so  by  me  in  later  years.  The  limb  of  the  planet, 
seen  by  these  observers  through  the  whole  width  of  the  dusky  ring 
in  1850,  could  not  be  traced  through  its  outer  half  by  myself  in  1872 


88  THE    TROUVELOT 

and  1874,  and  this  with  the  very  same  instrument  used  by  Bond  in 
his  observations  of  1848  and  1850.  Moreover,  I  have  plainly  seen 
that  its  transparency  was  not  everywhere  equal,  but  greatest  on  the 
inner  border,  from  which  it  gradually  decreases,  until  it  becomes 
opaque,  as  proved  by  the  gradual  loss  of  distinctness  of  the  limb, 
which  vanishes  at  about  the  middle  of  the  dusky  ring.  These  facts, 
which  have  been  well  ascertained,  prove  that  the  particles  compos- 
ing this  ring  are  not  permanently  located,  and  are  undergoing 
changes  of  relative  position.  It  will  be  shown  that  the  surface  of 
the  other  rings  is  also  subject  to  changes,  which  are  sometimes  very 
rapid. 

The  globe  of  Saturn  is  not  self-luminous,  but  opaque.  It  shines 
by  the  solar  light,  as  is  proved  by  the  shadow  it  casts  opposite  the 
Sun  upon  the  ring.  Although  receiving  its  light  from  the  Sun, 
Saturn  does  not  exhibit  any  traces  of  phases,  like  the  other  planets 
nearer  to  the  Sun,  owing  to  its  great  distance  from  the  Earth. 
When  near  its  quadratures,  however,  the  limb  opposite  to  the  Sun 
appears  much  darker,  and  shows  traces  of  twilight.  As  far  as  can 
be  ascertained,  the  rings,  with  the  exception  of  the  inner  one,  are 
opaque,  as  proved  by  the  strong  shadow  which  they  cast  on  the 
globe  of  Saturn. 

The  shadows  cast  by  the  planet  on  the  ring,  and  by  the  ring  on 
the  planet,  are  very  interesting  phenomena,  inasmuch  as  they  en- 
able the  astronomer  to  recognize  the  form  of  the  surface  which  re- 
ceives them.  The  shadow  cast  by  the  ring  on  the  ball  is  not  quite 
so  interesting  as  the  other,  although  it  has  served  to  prove  that  the 
surface  of  this  globe  is  not  smooth,  as  is  likewise  suggested  by  its 
mottled  appearance.  I  have  sometimes  found,  as  have  also  other 
observers,  that  the  outline  of  this  shadow  upon  the  ball  was  irregu- 
lar and  indented,  an  observation  which  proves  either  that  the  sur- 
face of  the  ball  is  irregular,  or  that  the  border  of  the  ring  casting 
the  shadow  was  jagged.  The  shadow  of  the  globe  on  the  rings  has 
much  more  interest,  as  it  enables  us  to  get  at  some  knowledge  of  the 
form  of  the  surface  of  the  rings,  which  otherwise  is  very  difficult  to 
discover,  owing  to  the  oblique  position  in  which  we  always  see  them. 

In  general,  the  shadow  of  the  ball  on  the  middle  ring  has  its  out- 
line concave  towards  the  planet;  while  on  the  outer  ring  it  is  usually 
slanting,  and  at  a  greater  distance  from  the  limb  than  on  the  mid- 
dle, and  dusky  rings.  This  form  of  the  shadow  evidently  proves 
that  the  middle  ring  stands  at  a  higher  level  than  the  two  others, 
especially  towards  its  outer  margin.  The  system  seems  to  increase 
gradually  in  thickness  from  the  inner  border  of  the  dusky  ring  to  the 


ASTRONOMICAL     DRAWINGS.  89 

vicinity  of  the  outer  margin  of  the  middle  ring,  after  which  it  rapidly 
diminishes  on  this  border,  while  the  surface  of  the  outer  ring  is  al- 
most level. 

But  this  surface  is  by  no  means  fixed,  as  its  form  sometimes 
changes,  as  proved  by  my  observations  and  those  of  others.  As 
may  be  noticed  on  Plate  X.,  the  outline  of  the  shadow  of  the  planet 
on  the  rings  is  strongly  deviated  towards  the  planet,  near  the  outer 
margin  of  the  middle  ring;  the  notch  indicating  an  abrupt  change  of 
level,  and  a  rise  of  the  surface  at  that  point.  Some  observers  have 
endeavored  to  explain  these  deviations  by  the  phenomena  of  irradi- 
ation, from  which  it  would  follow  that  the  maximum  effect  of  de- 
viation should  be  observed  where  the  ring  is  the  brightest,  which 
does  not  accord  with  observation  ;  as  the  deepest  depression  in 
the  shadow  is  not  to  be  found  usually  at  the  brightest  part,  which 
is  towards  the  outer  border  of  the  middle  ring,  but  occurs  near  its 
centre.  From  these  observations  it  is  undoubtedly  established  that 
the  surface  of  the  rings  is  far  from  being  flat  throughout,  and  is, 
besides,  not  permanent,  but  changes,  as  would,  for  instance,  the  sur- 
face of  a  large  mass  of  clouds  seen  from  the  top  of  a  high  mountain. 
In  general,  the  system  is  thickest  not  very  far  from  the  outer  border 
of  the  intermediary  ring. 

Some  interesting  phenomena  which  I  had  occasion  to  observe 
before  and  after  the  passage  of  the  Sun  through  the  plane  of  the 
rings,  on  February  6th,  1878,  conclusively  show  that  the  surface  of 
this  system  cannot  be  of  a  uniform  level,  but  must  be  thicker  towards 
the  outer  border  of  the  middle  ring,  thence  gradually  sloping  to- 
wards the  planet.  Many  of  my  observations  irresistibly  lead  to  this 
conclusion.  As  it  would,  however,  be  out  of  place  to  have  them 
recorded  here  in  detail,  I  will  simply  give  one  of  the  most  character- 
istic among  them. 

From  December  i8th,  1877,  when  the  Sun  was  about  41'  above 
the  plane  of  the  rings,  to  February  6th,  1878,  the  day  of  its  passage 
through  their  plane,  the  illuminated  surface  of  this  system  gradually 
decreased  in  breadth  with  the  lowering  of  the  Sun,  until  it  was  lost 
sight  of,  February  5th,  on  the  eve  of  the  passage  of  the  Sun  through 
their  plane.  The  phenomenon  in  question  consisted  in  the  gradual 
invasion  of  their  illuminated  surface  by  what  appeared  to  be  a  black 
shadow,  apparently  cast  by  the  front  part  of  the  outer  portion  of  the 
middle  ring  the  nearest  to  the  Sun.  On  January  25th,  when  the 
elevation  of  the  Sun  above  the  plane  of  the  rings  was  reduced  to  15', 
the  shadow  thus  cast  had  extended  so  far  on  their  surface  that  it 
reached  the  shadow  cast  by  the  globe  on  the  opposite  part  of  the 


90  THE    TROUVELOT 

ring  in  the  east,  and  accordingly  the  remaining  portion  of  the  illu- 
minated surface  of  the  eastern  ansa  then  appeared  entirely  discon- 
nected from  the  ball,  by  a  large  dark  gap,  corresponding  in  breadth 
to  that  of  the  globe's  shadow  on  the  rings.     On  February  4th,  when 
the  Sun  was  only  5'  above  the  plane  of  the  rings,  their  illuminated 
and  only  visible  surface  was  reduced  to  a  mere  thread  of  light,  which 
on  the  5th  appeared  broken  into  separate  points.    It  is  evident  that 
the  phenomenon  was  not  caused  by  the  obliquity  of  the  ring  as  seen 
from  our  globe,  since  the  elevation  of  the  Earth  above  the  plane  of 
the  rings — which  on  December  i8th  was  3°  20' — was  still  i°  20'  on  the 
4th  of  February.     In  ordinary  circumstances,  when  the  Sun  is  a  lit- 
tle more  elevated,  and  the  rings  seen  at  this  last  angle,  they  appear 
quite  broad  and  conspicuous,  and  even  the  dark  open  space  separating 
the  dusky  ring  from  the  planet  is  perfectly  visible  on  the  ansae,  where 
the  Earth's  elevation  above  their  plane  is  reduced  to  40'.     It  is  also 
evident  that  the  phenomenon  was  not  to  be  attributed  to  the  reduc- 
tion of  the  light  which  they  received  from  the  Sun,  although  the  illu- 
mination in  February  might  be  expected  to  be  comparatively  feeble, 
since  the  Sun  then  shone  upon  the  rings  so  obliquely;  yet  (on  the  sup- 
position that  their  surface  is  flat)  they  should  have  been  illuminated 
throughout,  and  if  not  very  brightly,  sufficiently  so,  at  least,  to  make 
them  visible  and  as  bright   as  was   the   narrow   thread   of  light   ob- 
served on  the  4th  of  February.     The  phenomenon  actually  observed 
may  be  explained  most  readily  by  assuming,  as  other  phenomena  also 
indicate,  that  the  surface  of  the  ring  is  not  flat,  but  more  elevated 
towards,  or  in  the  vicinity  of  its  outer  border,  from  which  place  it 
slopes  inwardly  towards  the  planet.     On  this  assumption,  it  is  evi- 
dent that  the  elevated  part  of  the  ring  the  nearest  to  the  Sun  would 
cast  a  shadow,  which,  with  the  increasing  obliquity  of  the  Sun,  would 
gradually  cover  the  whole   surface  comprised  within  the   elevated 
part,  and  thus  become  invisible  to  us.     Several   observations  made 
by  Bond  and  other  observers  undoubtedly  show  the  same  phenome- 
non, and  do  not  seem  to  be  intelligible  on   any  other  supposition. 
From  my  observations  made  in   1881  it  would  appear,  however,  that 
the  opposite  surfaces  of  the  rings  do  not  exactly  correspond  in  form, 
but  this  may  not  be  a  permanent  feature,  as  the  surface  of  this  sys- 
tem is  subject  to  changes,  as  already  shown. 

The  dimensions  of  the  rings  are  great,  the  diameter  of  the  outer 
one  being  no  less  than  172,982  miles,  the  distance  from  the  centre 
of  the  globe  to  the  outer  border  of  the  system  being,  therefore, 
86,491  miles.  The  breadth  of  the  outer  ring  is  9,941  miles  ;  that  of 
the  principal  division,  2,131  miles  ;  that  of  the  middle  ring,  19,902 


ASTRONOMICAL    DRA  WINGS.  91 

miles,  and  that  of  the  dusky  ring,  8,772  miles.  The  breadth  of  all 
the  rings  taken  together  is,  therefore,  40,746  miles.  The  interval 
between  the  surface  of  Saturn  and  the  inner  border  of  the  dusky  ring 
is  7,843  miles. 

The  thickness  of  the  system  of  rings  has  been  variously  esti- 
mated by  astronomers,  on  account  of  the  great  difficulties  attending 
its  determination.  While  Sir  John  Herschel  estimated  it  at  more 
than  250  miles,  G.  P.  Bond  reduces  it  to  40  miles.  Both  of  these 
numbers  are  evidently  too  small,  as  so  slight  a  thickness  cannot  ex- 
plain the  observed  phenomenon  of  the  shadow  cast  by  a  portion  of 
the  ring  on  its  own  surface,  when  the  Sun  is  very  low  in  its  horizon, 
as  shown  above. 

The  plane  of  the  system  of  rings  is  inclined  27°  to  the  planet's 
orbit,  and  is  parallel,  or  at  least  very  nearly  so,  with  the  equator  of 
the  planet,  passing,  therefore,  through  its  centre,  and  dividing  its 
globe  into  northern  and  southern  hemispheres.  Seen  from  the  Earth, 
a  portion  of  the  ring  always  appears  projected  in  front  of  the  planet, 
thus  concealing  a  small  part  of  its  globe,  while  the  opposite  portion 
passes  behind  the  globe,  which  hides  it  from  sight. 

As  the  plane  of  the  ring  is  not  affected  by  the  motion  of  the 
planet  around  the  Sun,  but  always  remains  parallel  to  itself,  it  fol- 
lows that  as  Saturn  advances  in  its  orbit  the  rings  must  successive- 
ly present  themselves  to  us  under  various  angles  of  inclination,  ap- 
pearing, therefore,  more  or  less  elliptical,  and  presenting  two  maxi- 
ma and  two  minima  of  inclination  in  the  course  of  one  of  its  revolu- 
tions. As  the  revolution  of  Saturn  is  accomplished  in  29}^  years, 
the  maxima  and  the  minima  must  recur  every  14  years  and  9 
months  ;  the  maxima  being  separated  from  the  minima  by  an  in- 
terval of  7  years  and  4%  months. 

When  Saturn  arrives  at  the  two  opposite  points  of  its  orbit, 
where  the  major  axis,  of  its  ring  is  at  right  angles  to  the  line  joining 
its  centre  to  that  of  the  Sun,  the  ring,  which  is  then  viewed  at  an 
inclination  of  27°,  the  greatest  angle  at  which  it  can  ever  be  seen, 
has  reached  its  maximum  opening,  the  smaller  diameter  of  its  ellipse 
being  then  about  half  that  of  the  larger.  At  this  moment  the  out- 
er ring  projects  north  and  south  beyond  the  globe,  which  is  then 
completely  enclosed  in  its  ellipse.  The  maximum  opening  of  the 
northern  surface  of  the  ring  takes  place,  at  present,  when  Saturn  ar- 
rives in  longitude  262°,  in  the  constellation  Sagittarius,  and  that 
of  the  southern  surface  when  it  arrives  in  longitude  82°  in  the  con- 
stellation Taurus.  When,  on  the  contrary,  Saturn  reaches  the  two 
opposite  points  of  its  orbit,  where  the  plane  of  its  ring  is  parallel  to 


92  THE     TROUVELOT 

the  line  joining  its  centre  to  that  of  the  Sun,  the  opening  vanishes, 
as  only  the  thin  edge  of  the  ring  is  then  presented  to  the  Sun  and 
receives  its  light,  the  rest  being  in  darkness.  At  this  moment  the 
ring  disappears,  except  in  the  largest  telescopes,  where  it  is  seen  as 
an  exceedingly  thin  thread  of  light ;  and  the  Saturnian  globe,  hav- 
ing apparently  lost  its  ring,  appears  solitary  in  the  sky,  like  the  other 
planets.  The  disappearance  of  the  ring  from  this  cause  occurs  now 
when  Saturn  arrives  at  90°  from  either  of  the  positions  of  maximum 
inclination,  that  is,  in  longitude  352°  in  the  constellation  Pisces,  and 
in  longitude  172°  in  the  constellation  Leo. 

When  the  planet  is  in  any  other  position  than  one  of  these  last 
two,  either  the  northern  or  the  southern  surface  of  the  ring  is  illu- 
minated by  the  Sun,  while  the  opposite  surface  is  in  the  night,  and 
does  not  receive  any  direct  sunlight.  At  the  time  of  the  passage  of 
the  plane  of  the  ring  through  the  Sun's  centre,  a  change  takes  place 
in  the  illumination  of  the  ring.  If  it  is  the  northern  surface  which  has 
received  the  rays  of  the  Sun  during  the  previous  half  of  the  Saturnian 
year,  at  the  moment  the  plane  has  passed  the  centre  of  the  Sun,  the 
southern  surface,  after  having  been  buried  in  darkness  for  14^ 
years,  sees  the  dawn  of  its  long  day  of  the  same  length.  Such  a 
phenomenon  will  not  occur  until  1892,  when  the  passage  of  the  Sun 
from  the  northern  to  the  southern  side  of  the  ring  will  close  in  twi- 
light the  day  commenced  in  1878. 

Aside  from  the  periodic  disappearance  of  the  ring,  resulting  from 
the  passage  of  the  Sun  through  its  plane,  the  ring  may  also  disap- 
pear from  other  reasons.  Just  before  or  just  after  the  time  of  the 
passage  of  the  Sun  through  the  plane  of  the  ring,  the  Earth  and  the 
Sun  may  occupy  such  positions,  that  while  the  one  is  north  of  the 
plane  of  the  ring,  the  other  is  south  of  it,  or  vice  versa,  in  which 
event  the  ring  becomes  invisible,  because  its  dark  and  non-illumi- 
nated surface  is  presented  to  us.  The  ring  may  also  become  invisi- 
ble to  us  when  the  Earth  passes  through  its  plane. 

Since  the  distance  from  Saturn  to  the  Sun  is  to  the  distance  of 
the  Earth  from  this  last  body  as  9.54  is  to  I  ;  and  since  the  circum- 
ference of  a  circle  increases  in  the  same  proportion  as  its  radius,  it 
follows  that  the  diameter  of  the  Earth's  orbit  projected  on  the  orbit 
of  Saturn  would  occupy  only  ^V  part  of  the  latter,  or  about  12°  2'r 
this  being  6°  i'  on  either  side  of  the  nodes  of  the  rings.  To  de- 
scribe such  an  arc  on  its  orbit,  it  takes  Saturn  almost  360  days 
on  an  average,  or  almost  a  complete  year  ;  the  Earth  describing 
therefore  almost  a  whole  revolution  around  the  Sun  during  the  time 
it  takes  Saturn  to  advance  12°  2'  on  its  orbit.  Then,  when  Saturn 


ASTRONOMICAL    DRAWINGS.  93 

occupies  a  position  comprised  within  an  arc  6°  i'  from  either  side  of 
the  nodes  of  its  ring,  the  Earth,  by  its  motion,  is  liable  to  encoun- 
ter the  plane  of  the  ring,  when  therefore  it  will  only  present  its  thin 
edge  to  us,  and  becomes  invisible.  At  least  one  such  encounter  is 
unavoidable  within  the  time  during  which  Saturn  occupies  either  of 
these  positions  on  its  orbit  ;  while  three  frequently  happen,  and  two 
are  possible. 

The  natural  impression  received  by  looking  at  the  rings,  while 
seeing  the  ponderous  globe  of  Saturn  enclosed  in  its  interior,  is  that 
this  gigantic,  but  very  delicate  structure,  in  order  to  avoid  destruc- 
tion, must  be  endowed  with  a  swift  movement  of  rotation  on  an 
axis  perpendicular  to  its  plane,  and  that  the  centrifugal  force  thence 
arising  counterbalances  the  powerful  attraction  of  the  planet,  and 
thus  keeps  the  system  in  equilibrium. 

Theoretically,  the  rotation  of  the  rings  is  admitted  by  every 
astronomer,  as  being  an  essential  condition  to  the  existence  of  the 
system,  which  otherwise,  it  is  thought,  would  fall  upon  the  planet. 
Although  the  rotation  of  the  rings  seems  so  probable  that  it  is  theo- 
retically considered  as  certain,  yet  its  existence  has  not  been  satisfac- 
torily demonstrated  by  direct  observation,  which  alone  can  establish 
it  on  a  firm  basis  as  a  matter  of  scientific  knowledge. 

The  determination  of  the  period  of  rotation  of  the  rings,  which  is 
supposed  to  be  ich.  32m.  153.,  rests  only  on  the  observations  of  W. 
Herschel,  made  in  1790,  from  the  apparent  displacement  of  irregu- 
larities on  the  ring  ;  but  his  results  have  been  contradicted  by  other 
observations,  and  even  by  those  of  Herschel  himself,  made  in  later 
years. 

Although  the  system  of  rings  is  very  nearly  concentric  with  the 
globe  of  Saturn,  yet  the  coincidence  is  not  considered  as  mathemat- 
ically exact.  It  seems  to  have  been  satisfactorily  demonstrated  by 
direct  observations  that  the  centre  of  gravity  of  the  system  oscillates 
around  that  of  the  planet,  thus  describing  a  minute  orbit.  This 
peculiarity  is  in  accordance  with  theory,  which  has  shown  it  to  be 
essential  to  the  stability  of  the  system. 

Besides  its  system  of  rings,  which  makes  Saturn  the  most  remarka- 
ble planet  of  the  solar  system,  this  globe  is  attended  by  eight  satel- 
lites, moving  in  orbits  whose  planes  very  nearly  coincide  with  the  plane 
of  the  rings,  except  that  of  the  most  distant  one,  which  has  an  inclina- 
tion of  about  12°  14'.  In  the  order  of  their  distance  from  the  planets, 
the  satellites  of  Saturn  are  as  follows  :  Mimas,  Enceladus,  Tethys, 
Dione,  Rhea,  Titan,  Hyperion  and  lapetus.  The  three  first  satellites- 
are  nearer  to  Saturn  than  the  Moon  is  to  the  Earth  ;  while  lapetus, 


94  THE    TROUVELOT 

the  farthest,  is  9^  times  the  distance  of  our  satellite  from  us.  All 
the  satellites,  with  the  exception  of  the  farthest,  move  more  rapidly 
around  Saturn  than  the  Moon  moves  around  the  Earth  ;  while 
lapetus,  on  the  contrary,  takes  almost  three  times  as  long  to  make 
one  revolution. 

The  period  of  revolution  of  the  four  inner  satellites  is  accom- 
plished in  less  than  three  days,  that  of  Mimas  being  only  a  little 
more  than  22  hours.  From  such  swiftness  of  motion,  it  is  easily 
understood  how  short  must  be  the  intervals  between  the  different 
phases  of  these  satellites.  Mimas,  for  instance,  passes  from  New 
Moon  to  First  Quarter  in  less  than  6  hours. 

The  distance  of  the  nearest  satellite  from  the  planet's  surface  is 
84,000  miles,  and  its  distance  from  the  outer  ring  only  36,000  miles. 
It  is  difficult  to  determine  the  diameter  of  objects  so  faint  and  dis- 
tant as  are  some  of  these  satellites,  but  the  diameter  of  Titan,  the 
largest  of  all,  is  pretty  well  known,  and  estimated  to  be  TV  the  dia- 
meter of  the  planet,  or  more  than  half  the  diameter  of  our  globe. 

lapetus  is  subject  to  considerable  variations  in  brilliancy,  and  as 
the  maxima  and  minima  always  occur  when  this  satellite  occupies 
the  same  parts  of  its  orbit,  it  was  conjectured  by  W.  Herschel  that, 
like  our  Moon,  it  turns  once  upon  its  axis  during  each  of  its  revolu- 
tions about  the  planet.  It  has  been  shown  by  my  observations, 
that  lapetus  attains  its  maximum  brightness  a  little  before  it  reaches 
its  greatest  western  elongation,  and  its  minimum  on  the  opposite 
side. 

As  the  planes  of  the  orbits  of  the  satellites  are  inclined  to  the 
planet's  orbit,  it  follows  that  their  transits,  occultationsand  eclipses, 
are  only  possible  when  Saturn  is  near  its  equinoxes.  Passages  of  the 
satellites  and  their  shadows  across  the  disk,  although  rare,  have 
been  observed,  and  they  somewhat  resemble  the  phenomena  exhib- 
ited by  the  satellites  of  Jupiter  in  transit.  When  the  Earth  is  very 
near  the  plane  of  the  rings,  the  satellites,  except  the  farthest, 
appear  to  be  in  a  straight  line  nearly  coincident  with  the  plane  of 
the  rings,  and  are  seen  occasionally  moving  along  the  thin  edge  of 
the  rings,  appearing  as  luminous  beads  moving  on  a  thread  of  light. 
Owing  to  the  considerable  inclination  of  the  axis  of  rotation  of 
Saturn  to  its  orbit,  the  seasons  of  this  planet  must  have  greater  ex- 
tremes of  temperature  than  those  of  the  Earth.  As  the  year  of 
Saturn  consists  of  25,217  Saturnian  days,  each  season,  on  the  aver- 
age, is  composed  of  6,304  Saturnian  days. 

To  an  observer  on  Saturn,  the  immense  arches  formed  by  its 
rings  would  appear  as  objects  of  great  magnificence,  spanning  the 


ASTRONOMICAL    DRA  WINGS.  95 

sky  like  soft  colorless  rainbows.  Moreover,  the  eight  moons,  several 
of  which  are  always  visible,  would  be  of  the  highest  interest,  with 
their  swift  motions  and  rapid  phases.  Mimas,  traveling  in  its  orbit 
at  the  rate  of  16'  of  arc  per  minute  of  time,  moves  over  a  space  equal 
to  the  apparent  diameter  of  our  Moon  in  two  minutes,  or  at  the  rate 
of  16°  an  hour. 

Owing  to  the  globular  form  of  Saturn,  the  rings  would  be  invisi- 
ble in  latitudes  situated  above  65°  from  its  equator,  and  their  ap- 
parent form  and  breadth  would  naturally  vary  with  the  latitude. 
At  63°  only  a  very  small  portion  of  the  outer  ring  would  be  visible 
above  the  equatorial  horizon,  where  it  would  appear  as  a  small 
segment  of  a  circle.  At  62°  the  principal  division  would  just  graze 
the  horizon.  At  46°  the  outer  portion  of  the  dusky  ring  would 
become  visible,  while  at  35°  its  inner  edge  would  appear  above  the 
horizon.  From  65°  of  latitude  down  to  the  equator,  the  arches  of 
the  rings  would  be  seen  more  and  more  elevated  above  the  equato- 
rial horizon,  but  at  the  same  time  that  they  are  seen  higher  up, 
their  apparent  breadth  gradually  diminishes,  owing  to  the  effect  of 
foreshortening,  and  at  the  equator  itself  the  system  would  only  pre- 
sent its  thin  edge  to  view. 

During  the  summer  seasons  of  either  hemisphere  of  Saturn,  the 
surface  of  the  rings  turned  towards  such  hemisphere,  being  fully  illu- 
minated by  the  Sun,  is  visible  from  these  regions.  In  the  day  time 
its  light  must  be  feeble  and  similar  to  the  light  reflected  by  our 
Moon  during  sunshine  ;  but  at  night  the  system  would  display  all  its 
beauty,  and  the  different  rings,  with  their  divisions  and  their  various 
reflective  powers,  must  present  a  magnificent  sight. 

During  the  nights  of  the  long  winter  seasons  on  Saturn,  on  the 
contrary,  the  surface  of  the  rings  turned  towards  the  hemisphere 
undergoing  winter,  receives  no  light  from  the  Sun,  and  is  invisible,  or 
very  nearly  so,  except  towards  morning  and  evening,  when  it  may 
be  faintly  illuminated  by  the  secondary  light  which  it  receives  from 
the  illuminated  globe  of  Saturn.  Although  dark  and  invisible,  the 
rings  may  make  their  form  apparent  at  night  by  the  absence  of 
stars  from  the  region  which  they  occupy  in  the  sky.  Again,  in 
other  seasons,  the  days  present  very  curious  phenomena.  In  con- 
sequence of  the  diurnal  rotation  of  the  planet,  the  Sun  seems  to 
move  in  circular  arcs,  which,  owing  to  the  inclination  of  Saturn's 
axis,  are  more  or  less  elevated  above  its  horizon,  according  to  the 
position  of  the  planet  in  its  orbit.  As  such  arcs  described  by  the 
Sun  in  the  sky  of  Saturn  are  liable  to  encounter  the  rings,  the  Sun 
in  passing  behind  them  becomes  eclipsed.  It  must  be  a  magnificent 


96  THE     TROUVELOT 

spectacle  to  witness  the  gradual  disappearance  of  the  fiery  globe  be- 
hind the  outer  ring,  and  its  early  reappearance,  but  for  a  moment 
only,  through  the  narrow  gap  of  the  principal  division;  to  see  it  van- 
ish again  behind  the  middle  ring,  to  reappear  a  little  later  through  the 
semi-transparent  dusky  ring,  but  very  faint  and  red  colored  at  first; 
and  then,  gradually  brighten  up,  and  finally  emerge  in  all  its  beauty 
from  the  inner  edge  of  the  dusky  ring. 

It  is  in  latitude  23°  that  the  rings  produce  the  most  prolonged 
eclipses  of  the  Sun.  During  a  period  equivalent  to  ten  of  our  terres- 
trial years,  such  eclipses  continually  succeed  each  other  with  but 
very  short  periods  of  interruption;  and  even  during  a  long  series  of 
rotations  of  Saturn,  the  Sun  remains  completely  invisible  in  those 
regions  where  the  apparent  arcs  which  it  describes  coincide  with 
the  arcs  of  the  rings.  In  neighboring  latitudes,  the  eclipses  of  the 
Sun,  although  still  frequent,  would  have  a  shorter  and  shorter  dura- 
tion as  the  observer  should  travel  north  or  south.  These  eclipses 
of  the  Sun  must  produce  a  partial  darkness  of  the  regions  involved 
in  the  shadow  of  the  rings,  which  may  be  compared  to  the  darkness 
produced  on  our  globe  by  a  total  eclipse  of  the  Sun.  The  frequent 
recurrence  of  these  eclipses,  and  their  comparatively  long  duration 
in  some  regions,  must  still  further  reduce  the  duration  of  the  short 
Saturnian  days. 

The  globe  of  Saturn,  as  already  shown,  casts  a  shadow  on  the 
rings,  which,  according  to  the  position  of  the  planet  in  its  orbit, 
either  extends  across  their  whole  breadth,  or  covers  only  a  part  of 
their  surface.  The  shadow  on  the  rings  rising  in  the  east  after  sun- 
set, ascends  to  the  culminating  point  of  their  arcs  in  the  sky,  in  2h. 
34m.,  and  as  rapidly  descends  on  the  western  horizon,  to  disappear 
with  sunrise.  This  shadow,  when  projected  on  the  rings  in  the  sky, 
must  be  hardly  distinguishable  from  the  dark  background  of  the 
heavens,  except  from  the  absence  of  stars  in  the  regions  which  it 
occupies.  It  must  appear  as  a  large  dark  gap,  separating  the  rings 
into  two  parts,  and  constantly  moving  from  east  to  west.  Possibly 
the  refraction  of  the  solar  rays,  in  passing  through  Saturn's  atmo- 
sphere, may  cast  some  colored  light  on  the  rings,  similar  to  that 
observed  on  the  Moon  during  its  eclipses. 

An  observer  on  the  rings  would  behold  phenomena  still  more 
curious,  a  long  day  of  14^  years  being  followed  by  a  long  night  of 
14^  years.  The  long  days  of  Saturn's  rings  are,  however,  diversi- 
fied by  numerous  eclipses  of  the  Sun,  which  regularly  occur  every 
10^  hours;  the  phenomenon  being  due  to  the  interposition  of  the 
globe  of  Saturn  between  the  rings  and  the  Sun.  These  eclipses 


ASTRONOMICAL     DRA  WINGS.  97 

produce  partial  obscurations  of  their  surface,  lasting  from  \yz  to  2 
hours  at  a  time.  Although  the  surface  of  the  rings  never  receives 
direct  sunlight  during  their  long  nights,  yet  they  are  not  plunged 
all  the  time  in  total  darkness,  as  they  receive  some  reflected  light 
from  that  part  of  the  globe  of  Saturn  which  is  illuminated  by  the 
Sun.  To  the  supposed  observer  on  the  rings,  during  every  10^ 
hours,  the  immense  globe  would  exhibit  continually  changing  phases. 
At  first  he  would  see  a  point  of  light  rapidly  ascending  from  the  hori- 
zon, and  appearing  under  the  form  of  a  half  crescent  of  considerable 
radius;  5>^  hours  later,  the  crescent  "having  gradually  increased, 
would  appear  as  a  half  circle,  covering  ^  of  the  visible  heavens, 
its  surface  being  more  than  20,000  times  as  large  as  the  surface  of 
the  Moon.  Upon  this  brilliantly  illuminated  semi-circle  would  be 
projected  the  shadows  of  the  rings,  appearing  as  black  belts  separ- 
ated by  a  narrow  luminous  band. 

It  is  very  difficult  for  one  to  conceive  how  such  a  delicate  struc- 
ture, as  the  system  of  rings  appears  to  be,  can  keep  together  in  equi- 
librium and  avoid  destruction  from  the  powerful  attraction  of  the 
planet  on  one  side  and  the  disturbing  influence  of  the  satellites  on 
the  other.  To  explain  it,  several  hypotheses  have  been  advanced. 
The  rings  were  first  supposed  to  be  solid,  and  upon  this  supposition 
Laplace  determined  the  necessary  conditions  for  their  equilibrium;  the 
most  important  of  which  require  that  the  cross  section  of  the  rings 
should  be  an  ellipse  of  irregular  curvature,  and  having  its  major 
axis  directed  towards  the  centre  of  the  planet,  and  also  that  the 
system  should  rotate  upon  an  axis  perpendicular  to  the  plane 
of  the  rings.  This  theory  was  superseded  by  another,  which  sup- 
posed the  rings  to  be  fluid.  This  one  was  soon  rejected  for  a  third, 
assuming  the  system  to  be  composed  of  vapors  or  gases  ;  and  more 
recently,  all  these  theories  were  considered  untenable,  and  re- 
placed by  a  fourth,  which  supposes  the  system  of  rings  to  be  made 
up  of  a  congregation  of  innumerable  small,  independent  bodies, 
revolving  around  Saturn  in  concentric  zones.  Naturally,  such  a 
divergence  of  opinion  can  only  result  from  our  comparative  ignorance 
of  the  subject,  and  sufficiently  indicates  our  inability  to  explain  the 
phenomena  ;  and  it  must  be  admitted  that,  so  far,  nothing  is  cer- 
tainly known  about  this  strange  system.  We  shall  probably  remain 
in  the  same  uncertainty  until  the  rotation  of  the  rings  is  ascertained 
by  direct  observations.  It  is  pretty  certain,  however,  that  none  of 
these  theories  account  for  the  observed  phenomena  in  their  details, 
although  a  partial  explanation  may  be  obtained  by  borrowing  some- 
thing from  each  hypothesis. 


98  THE    TROUVELOT 

It  has  been  conjectured,  and  a  theory  has  been  advanced,  that 
the  breadth  of  the  whole  ring  system  is  gradually  increasing  in- 
wards, and  that  it  will  come  in  contact  with  the  planet  in  about 
2,150  years  ;  but  the  question  seems  to  have  been  settled  in  the 
negative  by  the  elaborate  measurements  of  the  English  observers. 
It  is  likely  that  the  increase  is  only  in  the  defining  power  of  the  in- 
struments. 


ASTRONOMICAL    DRAWINGS.  99* 


COMETS. 

PLATE  XL 

AMONG  the  celestial  phenomena,  none  are  more  interesting  than 
those  mysterious  apparitions  from  the  depths  which  unexpectedly 
display  their  strange  forms  in  our  familiar  constellations,  through 
which  they  wander  for  a  time,  until  they  disappear  like  phantoms. 
A  comet,  with  its  luminous  diffused  head,  whence  proceeds  a  long 
vapory  appendage  gradually  fading  away  in  the  sky,  presents  an 
extraordinary  aspect,  which  may  well  astonish  and  deeply  impress 
the  observer.  Although  these  visitors  from  infinite  space  do  not 
now  inspire  dread,  as  in  by-gone  times,  yet,  owing  to  the  mystery 
in  which  the  phenomenon  is  still  involved,  the  apparition  of  a  large 
comet,  even  in  our  days,  never  fails  to  create  a  profound  sensation, 
and  in  some  cases  that  unconscious  fear  which  results  from  the 
unknown. 

The  effect  of  such  a  spectacle  largely  depends  upon  its  rarity  ; 
but  since  the  telescope  has  been  applied  to  the  sounding  of  the 
heavens,  it  has  been  found  that  the  appearance  of  comets  is  by  no 
means  an  unusual  occurrence.  If  so  few  comets,  comparatively,  are 
seen,  it  is  because  most  of  them  are  telescopic  objects,  and  are  there- 
fore invisible  to  the  naked  eye.  Most  of  the  telescopic  comets  are 
not  only  too  faint  to  be  perceived  by  the  unaided  eye,  but  are  insig- 
nificant objects,  even  when  observed  through  the  largest  telescopes. 

It  was  Kepler's  opinion  that  comets  are  as  numerous  in  the  sky 
as  fishes  are  in  the  ocean.  Undoubtedly  the  number  of  these  bodies 
must  be  great,  considering  that  we  can  only  see  them  when  they 
come  into  the  neighborhood  of  the  Earth,  and  that  many  even  here 
remain  invisible,  or  at  least  pass  unperceived.  That  many  of  them 
have  passed  unperceived  heretofore,  is  proved  by  the  fact  that  the 
number  of  those  observed  becomes  greater  every  year,  with  the  in- 
crease of  the  number  of  instruments  used  in  their  search.  The  num- 
ber of  comets  observed  with  the  naked  eye  during  historic  times 
is  nearly  600,  and  that  of  telescopic  comets,  which,  of  course,  all 


100  THE    TROUVELOT 

belong  to  the  last  few  centuries,  is  more  than  200,  so  that  we  have 
a  total  number  of  about  800  comets  of  which  records  have  been 
kept.  From  theoretical  considerations,  Lambert  and  Arago  esti- 
mated their  entire  number  at  several  millions,  but  such  speculations 
have  generally  no  real  value,  since  they  cannot  be  established  on  a 
firm  basis. 

Comets  remain  visible  for  more  or  less  time,  according  to  their 
size  and  the  nature  and  position  of  their  orbits,  but  in  general,  the 
large  ones  can  be  followed  with  the  telescope  for  several  months 
after  they  have  become  invisible  to  the  naked  eye.  The  comet  of 
1861,  for  example,  remained  telescopically  visible  for  a  year,  and 
that  of  1 8 1 1,  for  17  months  after  disappearing  from  ordinary  sight. 

While  a  comet  remains  visible,  it  appears  to  revolve  daily  about 
us  like  the  stars  in  general  ;  but  it  also  moves  among  the  constella- 
tions, and  from  this  movement  its  orbit  may  be  computed  like  that 
of  a  planet.  From  the  apparent  diurnal  motion  of  a  comet  with  the 
heavens,  result  the  changes  of  position  which  it  seems  to  undergo 
in  the  course  of  a  night.  The  direction  of  the  head  and  tail  of  a 
comet,  of  course,  has  only  changed  in  regard  to  the  horizon,  but 
not  in  regard  to  the  sky,  in  which  they  occupy  very  nearly  the  same 
position  throughout  a  given  night,  and  even  for  many  nights  in  suc- 
cession. 

The  movements  of  the  comets  in  their  orbits  are,  like  those  of 
the  planets,  in  accordance  with  Kepler's  laws,  the  Sun  occupying  one 
of  the  foci  of  the  orbit  they  describe  ;  but  the  orbits  of  comets  differ, 
however,  in  several  points  from  those  of  the  planets.  Their  eccen- 
tricity is  always  great,  being  sometimes  apparently  infinite,  in  which 
case  the  orbit  is  said  to  be  parabolic,  or  hyperbolic;  but  the  smallness 
of  the  portion  of  a  cometary  orbit  which  can  ordinarily  be  observed, 
makes  it  difficult  to  determine  this  with  certainty.  Again,  while  the 
planetary  orbits  are  usually  near  the  plane  of  the  ecliptic,  those  of 
comets  frequently  have  great  inclinations  to  that  plane,  and  even 
when  the  inclination  is  less  than  90°,  the  comet  may  have  a  retro- 
grade movement,  or,  in  other  words,  a  movement  contrary  to  the 
course  in  which  all  the  planets  revolve  about  the  Sun. 

Notwithstanding  these  differences  between  the  elements  of  the 
orbits  of  the  comets  and  those  of  the  planets,  the  fact  that  each  has 
the  Sun  in  one  focus  indicates  that  the  body  moving  in  it  is  a  mem- 
ber of  the  solar  system,  either  for  the  time,  or  permanently,  accord- 
ing to  the  nature  of  its  orbit. 

A  distinction  may  accordingly  be  made  between  the  comets 
which  are  permanent  members  of  our  solar  system  and  those  which 


ASTRONOMICAL    DRAWINGS.  101 

are  only  accidental  or  temporary  visitors.  Those  moving  in  ellipti- 
cal orbits  around  the  Sun,  like  the  planets,  and  therefore  having  a 
determinate  period  of  revolution,  from  which  the  time  of  their  suc- 
cessive returns  may  be  predicted,  are  permanent  members  of  our 
system,  and  are  called  periodic  comets.  All  comets  moving  in  para- 
bolical or  hyperbolical  curves,  are  only  temporary  members  of  the 
solar  system,  being  apparently  strangers  who  have  been  diverted 
from  their  courses  by  some  disturbing  influence.  No  comet  is  classed 
as  periodical  which  does  not  follow  a  perceptibly  elliptical  orbit.  Any 
comet  passing  around  the  Sun  at  the  mean  distance  of  the  Earth 
from  this  body,  with  a  velocity  of  26  miles  per  second,  will  fly  off 
into  infinite  space,  to  return  to  us  no  more. 

The  time  of  revolution  of  the  different  periodic  comets  thus  far 
observed  varies  greatly,  as  do  also  the  distances  to  which  they  re- 
cede from  the  Sun  at  aphelion.  Whilst  the  period  of  revolution  of 
Encke's  comet,  the  shortest  thus  far  known,  is  only  3^2  years,  that  of 
the  comet  of  1844,  II.,  is  102,000  years  ;  and  whilst  the  orbit  of  the 
first  is  comprised  within  the  orbit  of  Jupiter,  that  of  the  last  extends 
to  a  distance  equal  to  147  times  the  distance  of  Neptune  from  the 
Sun.  But  so  vast  an  orbit  cannot  be  accurately  determined  from 
the  imperfect  data  at  our  disposal. 

The  periodic  comets  are  usually  divided  into  two  classes.  The 
comets  whose  orbits  are  within  the  orbit  of  Neptune  are  called  in- 
terior comets,  while  those  whose  orbits  extend  beyond  that  of  Nep- 
tune are  called  exterior  comets.  The  known  interior  periodic  com- 
ets are  twelve  in  number,  while,  including  all  the  cases  in  which 
there  is  some  slight  evidence  of  elliptic  motion,  the  number  of  ex- 
terior comets  observed  is  six  or  seven  times  as  great.  The  periodic 
comets  of  short  period  are  very  interesting  objects,  inasmuch  as  by 
their  successive  returns  they  afford  an  opportunity  to  calculate  their 
motions  and  to  observe  the  physical  changes  which  they  undergo 
in  their  intervals  of  absence. 

From  observation  of  the  periodic  comets,  it  has  been  learned 
that  the  same  comet  never  presents  twice  the  same  physical  appear- 
ance at  its  different  returns,  its  size,  shape  and  brilliancy  varying  so 
greatly  that  a  comet  can  never  be  identified  by  its  physical  charac- 
ters alone.  It  is  only  when  its  elements  have  been  calculated,  and 
are  found  to  agree  with  those  of  a  cometary  orbit  previously  known, 
that  the  two  comets  can  be  identified  one  with  the  other.  There 
are  reasons  to  believe  that,  in  general,  comets  decrease  in  bright- 
ness and  size  at  each  of  their  successive  returns,  and  that  they  are 
also  continually  losing  some  of  their  matter  as  they  traverse  their 
orbits. 


102  THE     TROUVELOT 

When  very  far  away  from  us,  all  comets  appear  nearly  alike,  con- 
sisting of  a  faint  nebulosity,  of  varying  dimensions.  When  a  comet 
first  appears  in  the  depths  of  space,  and  travels  towards  the  Sun,  it 
generally  resembles  a  faint,  uniformly  luminous  nebulosity,  either 
circular  or  slightly  elongated  in  form.  As  it  approaches  nearer  to 
the  Sun,  a  slight  condensation  of  light  appears  towards  its  centre, 
and  as  it  draws  still  nearer,  it  becomes  brighter  and  brighter,  and  in 
condensing  forms  a  kind  of  diffused  luminous  nucleus.  At  the  same 
time  that  the  comet  acquires  this  concentration  of  light,  the  nebu- 
losity gradually  becomes  elongated  in  the  direction  of  the  Sun. 
These  effects  generally  go  on  increasing  so  long  as  the  comet  is 
approaching  the  Sun  ;  the  condensation  of  light  sometimes  forms  a 
bright  nucleus,  comparable  to  a  very  brilliant  star,  while  the  elonga- 
tion becomes  an  immense  appendage  or  tail.  When  the  comet  has 
passed  its  perihelion  and  recedes  from  the  Sun,  the  inverse  phenom- 
ena are  observed  ;  the  comet,  decreasing  in  brightness,  gradually 
loses  its  nucleus  and  tail,  resumes  its  nebulous  aspect,  and  finally 
vanishes  in  space,  to  appear  again  in  due  course,  if  it  chance  to  be  a 
periodic  comet.  While  all  comets  become  brighter  in  approaching 
the  Sun,  they  do  not  all,  however,  develop  a  large  tail,  some  of 
them  showing  only  a  slight  elongation. 

When  a  comet  is  first  discovered  with  the  telescope  at  a  great 
distance  from  the  Sun,  it  is  difficult  to  predict  whether  it  will  be- 
come visible  to  the  naked  eye,  or  will  remain  a  telescopic  object,  as 
it  is  only  in  approaching  the  Sun  that  these  singular  bodies  acquire 
their  full  development.  Thus,  Donati's  comet,  whose  tail  became 
so  conspicuous  an  object  at  its  full  appearance  in  1858,  remained  two 
months  after  its  discovery  by  the  telescope  without  any  indication 
of  a  tail.  The  comet  of  Halley,  which  before  and  after  its  return  in 
1759,  remained  five  years  inside  of  the  orbit  of  Saturn,  showed  not 
the  least  trace  of  its  presence  during  the  greater  part  of  this  time. 
Nothing  but  calculation  could  then  indicate  the  position  in  the  sky 
of  this  invisible  object,  which  was  so  prominent  when  it  approached 
the  Sun. 

Another  curious  phenomenon  exhibited  by  comets,  and  first  no- 
ticed by  Valz,  is  that  in  approaching  the  Sun  the  nebulosity  com- 
posing these  bodies  contracts,  instead  of  dilating,  as  would  be 
naturally  supposed  from  the  greater  amount  of  solar  heat  -which 
they  must  then  receive.  In  receding  from  the  Sun,  on  the  contrary, 
they  expand  gradually.  As  comets  approach  the  Sun,  the  tail  and 
nucleus  are  developed,  while  the  nebulosity  originally  constituting 
these  comets  contracts,  as  if  its  material  had  been  partly  consumed 


ASTRONOMICAL     DRAWINGS.  103 

in  this  development.  In  a  certain  sense  it  may  be  said  that  the 
comets  are  partly  created  by  the  Sun;  in  more  exact  terms,  the 
changes  of  form  which  they  undergo  are  induced  by  the  Sun's  action 
upon  them  at  different  distances  and  under  varying  conditions. 
Moreover,  they  are  rendered  visible  by  its  influence,  without  which 
they  would  pass  unperceived  in  our  sky.  When  a  comet  disappears 
from  view,  it  is  not  because  its  apparent  diameter  is  so  much  reduced 
by  the  distance  that  it  vanishes,  but  rather  on  account  of  the  dim- 
inution of  its  light,  both  that  which  it  receives  from  the  Sun,  and 
its  own  light;  these  bodies  being  in  some  degree  self-luminous,  as 
will  be  shown  below. 

The  large  comets,  such  as  can  be  seen  with  the  naked  eye,  al- 
ways show  the  following  characteristics,  on  examination  with  the 
telescope.  A  condensation  of  light  resembling  a  diffused  star  forms 
the  brightest  part  of  the  comet,  this  condensation  being  situated 
towards  the  extremity  the  nearest  to  the  Sun.  It  is  this  starlike 
object  which  is  called  the  nucleus.  The  nucleus  seems  to  be  entirely 
enclosed  in  a  luminous  vapory  envelope  of  the  same  general  texture, 
called  the  coma.  This  envelope,  which  is  quite  variable  in  bright- 
ness and  form,  is  brightest  next  to  the  nucleus,  and  gradually  fades 
away  as  it  recedes  from  it.  The  nucleus  and  the  coma,  considered  as 
a  whole,  constitute  the  head  of  a  comet.  From  the  head  of  a  comet 
proceeds  a  long  trail  of  pale  nebulous  light,  which  usually  grows 
wider,  but  fainter,  as  it  recedes  from  the  nucleus,  and  insensibly 
vanishes  in  the  sky.  This  delicate  appendage,  or  tail,  as  it  is  com- 
monly called,  varies  very  much  in  size  and  shape,  not  only  in  differ- 
ent comets,  but  in  the  very  same  comet,  at  different  times.  Its 
direction  is  generally  opposite  to  that  of  the  Sun  from  the  head  of 
the  comet. 

The  nuclei  vary  very  much  in  brightness,  in  size  and  in  shape; 
and  while  in  some  telescopic  comets  they  are  either  absent  or  barely 
distinguishable  as  a  small  condensation  of  light,  in  bright  comets 
they  may  become  plainly  visible  to  the  naked  eye,  and  they  some- 
times even  surpass  in  brightness  the  most  brilliant  stars  of  the 
heavens.  But  whatever  may  be  the  size  of  cometary  nuclei,  they 
are  subject  to  sudden  and  rapid  changes,  and  vary  from  day  to  day. 
Sometimes  they  appear  exceedingly  brilliant  and  sharply  outlined, 
while  at  other  times  they  are  so  dim  and  diffused  that  they  are  hardly 
distinguishable  from  the  coma  of  which  they  seem  then  to  form  a  part. 

From  my  observations  upon  the  comets  which  have  appeared 
since  the  year  1873,  it  is  apparent  that  the  changes  in  the  nucleus, 
coma  and  tail,  are  due  to  a  solar  action,  which  contracts  or  expands 


104  THE    TROUVELOT 

these  objects  in  such  a  manner  that  the  nuclei  become  either  bright 
and  star-like,  or  dim  and  diffused,  in  a  very  short  time.  I  had  ex- 
cellent opportunity,  especially  in  the  two  large  comets  of  1881,  to 
observe  some  of  these  curious  changes,  a  description  of  which  will 
give  an  idea  of  their  extent  and  rapidity.  On  July  2d,  1881,  at  9 
o'clock,  the  nucleus  of  comet  1881,  III.,  which  is  represented  on  Plate 
XI.,  appeared  sharply  defined,  bright  and  considerably  flattened  cross- 
wise; but  half  an  hour  later  it  had  considerably  enlarged  and  had 
become  so  diffused  that  it  could  hardly  be  distinguished  from  the 
coma,  with  which  it  gradually  blended.  It  is  perhaps  worth  mention 
that,  at  the  time  this  last  observation  was  made,  an  aurora  borealis 
was  visible.  This  comet  1881,  III.,  underwent  other  very  import- 
ant changes  of  its  nucleus,  coma  and  tail.  On  June  25th,  the  nucleus, 
which  was  bright  and  clearly  defined,  was  ornamented  with  four 
bright  diverging  conical  wing's  of  light,  as  shown  on  Plate  XI.  On 
the  26th  these  luminous  wings  had  gone,  and  the  nucleus  appeared 
one-third  smaller.  On  the  28th  it  had  enlarged,  but  on  the  2Qth 
its  shape  was  considerably  altered,  the  nucleus  extending  in  one 
direction  to  three  or  four  times  its  diameter  on  previous  nights,  and 
being  curved,  so  as  to  resemble  a  comma.  On  the  6th  of  July  the 
nucleus  of  this  comet  showed  the  greatest  disturbances.  The  nucleus, 
which  had  appeared  perfectly  round  on  the  evening  of  the  5th,  was 
found  much  elongated  at  10  o'clock  on  the  6th,  forming  then  a 
straight,  acute,  and  well-defined  wedge  of  light,  inclined  upwards  to 
the  left.  The  length  of  the  nucleus,  at  this  time,  was  three  or  four 
times  its  ordinary  diameter.  At  the  same  time  rapid  changes  oc- 
curred ;  the  strangely  shaped  nucleus  soon  became  unsteady,  ex- 
tending and  contracting  alternately,  and  varying  greatly  in  bright- 
ness. At  ich.  45m.,  the  elongated  nucleus,  then  gently  curved, 
took  the  shape  of  a  succession  of  luminous  knots,  which  at  times 
became  so  brilliant  and  distinct  that  they  seemed  to  be  about  to 
divide  and  form  separate  nuclei;  but  such  a  separation  did  not  actu- 
ally occur,  at  least  while  I  was  observing.  While  these  important 
changes  were  going  on  in  the  comet,  a  bright  auroral  arch  appeared 
in  the  north,  which  lasted  only  a  short  time.  On  July  /th,  the  sky 
being  cloudy,  no  observations  were  made,  but  on  the  8th  I  observed 
the  comet  again.  The  nucleus  had  then  resumed  its  circular  form,  but 
it  was  yet  very  unsteady,  being  sometimes  small,  bright  and  sharp, 
while  a  few  seconds  later  it  appeared  twice  as  large,  but  dim  in  out- 
lines; and  sometimes  an  ill-defined  secondary  nucleus  appeared  at 
its  centre.  On  several  occasions  the  nucleus  appeared  as  if  it  were 
double,  one  nucleus  being  apparently  projected  partly  upon  the  other. 


ASTRONOMICAL     DRAWINGS.  105 

The  nuclei  of  comets  are  sometimes  very  small,  and  in  other 
cases  very  large.  Among  those  which  have  been  measured,  the 
nucleus  of  the  comet  of  1798,  L,  was  only  28  miles  in  diameter,  but 
that  of  Donati's  comet,  in  1858,  was  5,600  miles,  and  that  of  the 
comet  of  1845  was  8,000  miles  in  diameter. 

The  coma  of  comets  is  found  to  be  even  more  variable  than  the 
nucleus.  The  changes  observed  in  the  coma  are  generally  in  close 
connection  with  those  of  the  nucleus  and  tail,  the  same  perturba- 
tions affecting  simultaneously  the  whole  comet.  While  the  coma 
of  the  comet  of  1847  was  only  18,000  miles  in  diameter,  that  of 
Halley's  comet,  in  1835,  was  357,000  miles,  and  that  of  the  comet  of 
1811  was  1,125,000  miles  in  diameter.  In  general,  as  already  stated, 
the  coma  of  a  comet  decreases  in  size  in  approaching  the  Sun.  That 
of  Encke's  comet,  which,  on  October  Qth,  1838,  had  a  diameter  of 
281,000  miles,  gradually  decreased  at  a  daily  mean  rate  of  4,088  miles 
in  going  towards  the  Sun  ;  so  that,  on  December  I7th,  when  the 
distance  of  the  cornet  from  the  Sun  was  more  than  four  times  less 
than  it  was  on  the  first  date,  its  diameter  was  reduced  to  3,000 
miles. 

The  form  of  the  coma,  in  that  part  which  is  free  from  the  tail,  is 
in  general  a  portion  of  a  circle,  but  is  sometimes  irregular,  with 
its  border  deformed.  Thus,  the  border  of  the  coma  of  Halley's 
comet  was  depressed  at  one  point  towards  the  Sun.  I  observed  a 
similar  phenomenon  in  Coggia's  comet,  with  the  great  refractor  of 
the  Harvard  College  Observatory,  on  July  I3th,  1874,  when  its 
border  appeared  deeply  depressed  on  the  side  nearest  to  the  Sun,  as 
if  repelled  by  this  body.  The  coma  of  comet  1881,  III.,  showed  also 
very  sigular  outlines  on  the  nights  of  the  25th  and  26th  of  June, 
when  its  border  was  so  deeply  depressed  that  the  coma  appeared  as 
if  it  were  double.  Luminous  rays  and  jets  often  radiate  from  the 
nucleus  across  the  coma,  and  describe  graceful  lateral  curves,  falling 
backwards  and  gradually  fading  away  into  the  tail,  of  which  they 
then  form  a  part.  The  rays  and  jets  emitted  by  the  nucleus  seem  at 
first  to  obey  the  solar  attraction  and  travel  towards  the  Sun  ;  but 
they  are  soon  repelled,  and  move  backward  towards  the  tail.  It  is  a 
mystery,  as  yet  unexplained,  how  these  cometary  jets,  which  at 
first  seem  to  obey  to  the  laws  of  attraction,  are  compelled  to  re- 
treat apparently  by  superior  opposing  forces.  Among  the  forces 
of  nature,  we  know  of  no  other  than  those  of  an  electrical  sort,  which 
would  act  in  a  similar  manner  ;  but  this  explanation  would  require 
us  to  assume  some  direct  electrical  communication  between  the 
comet  and  the  Sun.  Considering  the  distance  between  the  two 


106  THE    TROUVELOT 

bodies,  and  the  probable  absence  or  great  tenuity  of  the  gaseous 
material  in  interstellar  space,  such  an  assumption  is  a  difficult  one. 

Under  the  action  of  the  solar  forces,  the  c.oma  also  very  fre- 
quently forms  itself  into  concentric  luminous  arcs,  separated  by 
comparatively  dark  intervals.  These  luminous  semi-circles  vary  in 
number,  but  sometimes  there  are  as  many  as  four  or  five  at  a  time. 
All  great  comets  show  these  concentric  curves  more  or  less,  but 
sometimes  only  a  portion  is  visible,  the  rest  of  the  coma  having 
a  different  structure.  When  great  comets  approach  near  the  Sun, 
their  coma  is  generally  composed  of  two  distinct  parts,  an  inner  and 
an  outer  coma,  the  inner  one  being  due  to  the  luminous  jets  issu- 
ing from  the  nucleus,  which,  never  extending  very  far,  form  a  dis- 
tinct, bright  zone  within  the  fainter  exterior  coma. 

The  tails  of  comets,  which  are  in  fact  a  prolongation  of  the  comar 
are  likewise  extremely  variable  in  form.  They  are  sometimes  straight 
like  a  rod;  again,  are  curved  like  a  sabre,  or  even  crooked  like  an  S, 
as  was  that  of  the  comet  of  1769.  They  are  also  fan-shaped,  point- 
ed, or  of  the  same  width  throughout.  Many  of  these  appendages 
appear  longitudinally  divided  through  their  middle  by  a  narrow, 
darkish  rift,  extending  from  the  nucleus  to  the  extremity.  This 
peculiarity  appears  in  the  comet  shown  on  Plate  XI.  Sometimes 
the  dark  rift  does  not  commence  near  the  nucleus,  but  at  some 
distance  from  it,  as  I  observed  in  the  case  of  comet  1881,  III.,  on 
June  26th.  This  dark  rift  is  not  a  permanent  feature  of  a  comet's 
tail,  but  may  be  visible  one  day  and  not  at  all  the  next.  Comet 
1 88 1,  III.,  which  had  shown  a  dark  rift  towards  the  end  of  June,  did 
not  exhibit  any  such  rift  during  July  and  August,  when,  on  the 
contrary,  its  tail  appeared  brighter  in  the  middle.  Coggia's  comet, 
which  showed  so  prominent  a  dark  rift  in  July,  1874,  had  none  on 
June  loth.  On  the  contrary,  the  tail  was  on  that  date  very  bright 
along  its  middle,  as  also  along  each  of  its  edges. 

The  tail  of  a  comet  does  not  invariably  point  directly  away  from 
the  Sun,  as  above  mentioned,  and  sometimes  the  deviation  is  con- 
siderable ;  for  instance,  the  tail  of  the  comet  of  1577  deviated  21° 
from  the  point  opposite  to  the  Sun. 

In  general,  the  tail  inclines  its  extremity  towards  the  regions  of 
space  which  it  has  just  left,  always  presenting  its  convex  border  to 
the  regions  towards  which  it  is  moving.  It  is  also  a  remarkable 
fact  that  this  convex  border,  moving  first  in  space,  always  appears 
brighter  and  sharper  than  the  opposite  one,  which  is  often  diffused. 
From  these  peculiarities  it  would  seem  that  in  moving  about  the 
Sun  the  comets  encounter  some  resistance  to  their  motion,  from  the 


ASTRONOMICAL    DRAWINGS.  107 

medium  through  which  they  pass,  and  that  this  resistance  is  suffi- 
cient to  curve  their  tails  away  from  the  course  in  which  they  move, 
and  to  crowd  their  particles  together  on  the  forward  side.  It  is  es- 
pecially when  they  approach  their  perihelion,  and  move  more  rapidly 
on  a  curve  of  a  shorter  radius,  that  the  comets'  tails  show  the  greatest 
curvature,  unless  their  position  in  regard  to  the  observer  prevents 
their  being  advantageously  seen.  The  tail  of  Donati's  comet  pre- 
sented a  fair  illustration  of  this  peculiarity,  its  curvature  having  aug- 
mented with  the  velocity  of  the  comet's  motion  about  the  Sun.  But 
possibly  this  phenomenon  has  another  cause,  and  may  be  found 
rather  in  the  solar  repulsion  which  acts  on  comets  and  is  not  instan- 
taneously propagated  throughout  their  mass. 

Although,  in  general,  comets  have  but  one  tail,  it  is  not  very 
rare  to  see  them  with  multiple  tails.  The  comets  of  1807  and  1843 
had  each  a  double  tail;  Donati's  comet,  in  1858,  showed  several  nar- 
row, long  rectilinear  rays,  issuing  from  its  abruptly  curved  tail.  The 
comet  of  1825  had  five  branches,  while  that  of  1744  exhibited  no 
less  than  six  distinct  tails  diverging  from  the  coma  at  various 
angles.  In  general  character  the  multiple  and  single  tails  are  simi- 
lar. When  a  comet  has  two  tails,  it  is  not  rare  for  the  second  to 
extend  in  the  general  direction  of  the  Sun,  as  was  the  case  with 
the  great  comet  of  1881,  III.,  represented  on  Plate  XL  From  July 
I4th  to  the  2 1st  it  exhibited  quite  an  extended  conical  tail,  starting 
obliquely  downwards  from  the  right  side  of  the  coma,  and  directed 
towards  the  Sun.  From  the  24th  of  July  to  the  2d  of  August  this 
secondary  tail  was  exactly  opposite  in  its  direction  from  that  of  the 
primary  tail,  and  gave  to  the  head  a  very  elongated  appearance. 
Comet  1 88 1,  IV.,  also  exhibited  a  secondary  appendage,  not  directed 
towards  the  Sun,  but  making  an  angle  of  about  45°  with  the  main  tail. 

These  cometary  appendages  sometimes  attain  prodigious  dimen- 
sions. The  comets  of  1680  and  1769  had  tails  so  extended  that, 
after  their  heads  had  set  under  the  horizon,  the  extremities  of  these 
immense  appendages  were  still  seen  as  far  up  as  the  zenith.  In  a  sin- 
gle day  the  tail  of  the  comet  of  1843  extended  100°,  and  it  was 
thrust  from  the  comet  "  as  a  dart  of  light "  to  the  enormous  distance 
of  48,500,000  miles,  and  yet  of  this  immense  appendage  nothing  was 
left  on  the  following  day.  The  tail  of  Donati's  comet,  in  1858,  at- 
tained a  real  length  of  42,000,000  miles,  while  that  of  the  great 
comet  of  1843  nad  the  enormous  length  of  200,000,000  miles.  If 
this  last  comet  had  occupied  the  position  of  the  Sun,  which  it  ap- 
proached very  nearly  for  a  moment,  the  extremity  of  its  tail  would 
have  extended  60,000,000  miles  beyond  the  orbit  of  Mars. 


108  THE    TROUVELOT 

In  some  cases  the  tails  of  comets  have  been  seen  undulating  and 
vibrating  in  a  manner  similar  to  the  undulations  and  coruscations  of 
light  characteristic  of  some  auroras.  Many  observers  report  having 
seen  such  phenomena.  The  comet  of  1769  was  traversed  by  lumin- 
ous waves  and  pulsations,  comparable  to  those  seen  in  the  aurora 
borealis.  I  myself  observed  these  curious  undulations  in  Coggia's 
comet  in  1874,  while  the  head  of  this  object  was  below  the  horizon. 
For  an  hour  the  undulations  rapidly  succeeded  each  other,  and  ran 
along  the  whole  length  of  the  tail. 

Some  of  the  brightest  comets  have  shone  with  such  splendor  that 
they  could  be  observed  easily  in  full  sunshine.  Many  comets,  such 
as  those  of  1577  and  1744,  have  equaled  Sirius  and  Venus  in  bril- 
liancy. The  great  comet  of  1843,  which  suddenly  appeared  in  our 
sky,  was  so  brilliant  that  it  was  seen  by  many  observers  at  noon 
time,  within  a  few  degrees  from  the  Sun.  I  remember  that  I  myself 
saw  this  remarkable  object  .in  the  day  time,  with  a  number  of  per- 
sons, who  were  gazing  at  the  wonderful  apparition.  So  brilliant  was 
this  comet,  that  besides  its  nucleus  and  head,  a  portion  of  its  tail 
was  also  visible  in  the  day  time,  provided  the  observer  screened 
his  eyes  from  the  full  sunlight  by  standing  in  the  shadow  of  some 
building. 

Of  all  the  bodies  revolving  around  the  Sun,  none  have  been 
known  to  approach  so  near  its  surface  as  did  the  comet  of  1843. 
When  it  arrived  at  perihelion,  the  distance  from  the  centre  of  its 
nucleus  to  the  surface  of  the  Sun's  photosphere  was  only  96,000 
miles,  while  the  distance  from  surface  to  surface  was  less  than  60,000 
miles.  This  comet,  then,  went  through  the  solar  atmosphere,  and 
in  traversing  it  with  its  tremendous  velocity  of  366  miles  per  second, 
may  very  possibly  have  swept  through  some  solar  protuberances, 
many  of  which  attain  much  higher  elevations  than  that  at  which 
the  comet  passed.  The  comet  of  1680  also  approached  quite  near 
the  surface  of  the  Sun,  and  near  enough  to  encounter  some  of  the 
high  solar  protuberances,  its  distance  at  perihelion  being  about  two- 
thirds  of  the  Moon's  distance  from  the  Earth.  The  rapidity  of  motion 
of  the  comet  of  1843  was  such,  when  it  approached  the  Sun,  that  it 
swept  through  all  that  part  of  its  orbit  which  is  situated  north  of  the 
plane  of  the  ecliptic  in  a  little  more  than  two  hours,  moving  in  this 
short  time  from  one  noSe  to  the  other,  or  1 80°. 

But  if  some  comets  have  a  very  short  perihelion  distance,  that 
of  others  is  considerable.  Such  a  comet  was  that  of  1729,  whose 
perihelion  distance  was  383,000,000  miles,  the  perihelion  point  being 
situated  between  the  orbits  of  Mars  and  Jupiter. 


ASTRONOMICAL     DRA  WINGS.  109 

While  some  comets  come  near  enough  to  the  Sun  at  perihelion 
to  be  volatilized  by  its  intense  heat,  others  recede  so  far  from  it  at 
.aphelion  that  they  may  be  said  to  be  frozen.  The  shortest  comet- 
ary  aphelion  distance  known  is  that  of  Encke's  comet,  whose  great- 
est distance  from  the  sun  is  388,000,000  miles.  But  that  of  the 
•comet  of  1844  is  406,000,000,000  miles  from  the  Sun.  The  comets 
-of  1863  and  1864  are  so  remote  in  space  when  they  reach  their 
aphelion  points  that  light,  with  its  velocity  of  185,500  miles  a  sec- 
ond, would  require  171  days  in  the  first  case,  and  230  in  the  last,  to 
pass  from  them  to  the  Earth. 

The  period  of  revolution  of  different  comets  also  varies  immense- 
ly. While  that  of  Encke's  comet  is  only  3^  years,  that  of  comet 
1864,  II.,  is  280,000  years. 

Among  the  periodic  comets  of  short  period,  some  have  exhibited 
highly  interesting  phenomena.  Encke's  comet,  discovered  in  1818, 
is  remarkable  for  the  fact  that  its  period  of  revolution  diminishes  at 
each  of  its  successive  returns,  and  consequently  this  comet,  with  each 
revolution,  approaches  nearer  and  nearer  to  the  Sun.  The  decrease 
•of  the  period  is  about  2^/2,  hours  at  each  return.  Although  the  de- 
crease is  small,  if  it  go  on  in  future  as  it  does  at  present,  the  inevi- 
table consequence  will  be  that  this  comet  will  finally  fall  into  the 
Sun.  This  curious  phenomenon  of  retardation  has  been  attributed 
by  astronomers  to  the  existence  of  a  resisting  medium  filling  space, 
but  so  rare  and  ethereal  that  it  does  not  produce  any  sensible  effect 
•on  the  movements  of  the  planets.  But  some  other  causes  may  re- 
tard this  comet,  as  similar  retardations  have  not  been  observed  in  the 
case  of  other  periodic  comets  of  short  period.  These,  however,  are 
not  so  near  to  the  Sun,  and  perhaps  our  luminary  may  be  surround- 
ed by  matter  of  extreme  tenuity,  which  does  not  exist  at  a  greater 
distance  from  it. 

Another  of  the  periodic  comets  which  has  exhibited  a  very  re- 
markable phenomenon  of  transformation  is  Biela's  comet,  which  di- 
vided into  two  distinct  parts,  moving  together  in  the  same  direc- 
tion. When  this  comet  was  first  detected  at  its  return  in  1845,  it 
presented  nothing  unusual,  but  in  the  early  part  of  1846  it  was 
noticed  by  several  astronomers  to  be  divided  into  two  parts  of  un- 
equal brightness,  forming  thus  a  twin  comet.  At  its  next  return  in 
1852,  the  two  sister  comets  were  still  traveling  in  company,  but 
their  distance  apart,  which  in  1846  was  157,000  miles,  had  increased 
to  1,500,000  miles.  •  At  the  two  next  returns  in  1859  and  1865,  their 
position  not  being  very  favorably  situated  for  observation,  the 
•comets  were  not  seen.  In  1872  the  position  should  have  been  favor- 


110  THE    TROUVELOT 

able  for  observation,  and  they  were  consequently  searched  for,  but 
in  vain  ;  neither  comet  was  found.  An  astronomer  in  the  southern 
hemisphere,  however,  found  a  comet  on  the  track  of  Biela's,  but  cal- 
culation has  shown  that  the  two  objects  are  probably  not  identical, 
since  this  comet  was  two  months  behind  the  computed  position  for 
Biela's.  It  will  be  shown  in  the  following  chapter  that  our  globe 
probably  crossed  the  orbit  of  Biela's  comet  on  November  2/th,  1872, 
and  the  phenomena  resulting  from  this  passage  will  be  there  de- 
scribed. 

It  is  seen  from  these  observations  that  comets  may  be  lost  or 
dissipated  in  space  by  causes  entirely  unknown  to  us.  Biela's  comet 
is  not  the  only  one  which  has  been  thus  disintegrated.  Ancient 
historians  speak  of  the  separation  of  large  comets  into  two  or  more 
parts.  In  1661  Hevelius  observed  the  apparent  division  of  the  comet 
of  that  year  and  its  reduction  to  fragments.  The  return  of  this 
comet,  calculated  for  1790,  was  vainly  waited  for  ;  the  comet  was 
not  seen. 

Other  comets,  whose  periods  of  revolution  were  well  known, 
have  disappeared,  probably  never  to  return.  Such  is  Lexell's 
comet,  whose  period  was  5^  years  ;  also  De  Vice's  comet, 
both  of  which  are  now  lost.  It  is  supposed  that  Lexell's  comet, 
which  passed  twice  very  near  the  giant  planet  Jupiter,  had  its  or- 
bit changed  from  an  ellipse  to  a  parabola,  by  the  powerful  disturb- 
ing influence  of  this  planet,  and  was  thus  lost  from  our  system. 
Several  other  comets,  in  traveling  over  their  different  orbits,  have 
approached  near  enough  to  Saturn,  Jupiter  and  the  Earth  to  have 
their  orbits  decidedly  altered  by  the  powerful  attraction  of  these 
bodies. 

But  since  comets  are  liable  to  pass  near  the  planets,  and  several 
have  orbits  which  approach  that  of  the  Earth,  it  becomes  important 
for  us  to  know  whether  an  encounter  of  such  a  body  with  our  globe 
is  possible,  and  what  would  then  be  the  result  for  us.  Although 
that  knowledge  would  not  enable  us  to  modify  the  possibilities  of 
an  encounter,  yet  it  is  better  to  know  the  dangers  of  our  naviga- 
tion through  space  than  to  ignore  them.  This  question  of  a  collision 
of  the  Earth  with  a  comet  has  been  answered  in  different  ways,  ac- 
cording to  the  ideas  entertained  in  regard  to  the  mass  of  these 
bodies.  While  some  have  predicted  calamities  of  all  kinds,  such  as- 
deluges,  conflagrations,  or  the  reduction  of  the  Earth  to  incandes- 
cent gases,  others  have  asserted  that  it  would  produce  no  more 
effect  than  does  a  fly  on  encountering  a  railroad  train.  In  our  days- 
astronomers  entertain  very  little  fears  from  such  an  encounter,  be- 


ASTRONOMICAL    DRAWINGS.  Ill 

cause  the  probabilities  of  danger  from  an  occurrence  of  this  sort  are 
very  slight,  the  mass  of  an  ordinary  comet  being  so  small  com- 
pared with  that  of  our  globe.  We  know  with  certainty  that  the 
Earth  has  never  had  an  encounter  with  a  comet  by  which  it  has 
been  transformed  into  gases,  at  least  within  the  several  millions  of 
years  during  which  animal  and  vegetable  life  have  left  their  marks 
upon  the  stony  pages  of  its  history,  otherwise  these  marks  would 
not  now  be  seen.  If,  then,  such  an  accident  has  not  happened  dur- 
ing this  long  period,  the  chances  for  its  occurring  must  be  very 
small,  so  small  indeed  that  they  might  almost  be  left  out  of  the 
question.  It  is  true  that  our  globe  shows  signs  of  great  perturba- 
tions of  its  surface,  but  we  have  not  the  slightest  proofs  that  they 
resulted  from  an  encounter  with  a  celestial  body.  It  seems  very 
probable  that  our  globe  passed  through  the  tail  of  the  comet  of 
1861,  before  it  was  first  seen  on  June  2Qth  ;  but  nothing  unusual 
was  observed,  except  perhaps  some  phosphorescent  light  in  the  at- 
mosphere, which  was  afterwards  attributed  to  this  cause. 

The  density  and  mass  of  comets  must  be  comparatively  very 
small.  Their  tails  consist  of  matter  of  such  extreme  tenuity  that  it 
affects  but  very  little  the  light  of  the  small  stars  over  which  they 
pass.  The  coma  and  nucleus,  however,  are  not  quite  so  transparent, 
and  may  have  greater  masses.  On  several  occasions  I  have  seen  the 
light  of  stars  reduced  by  the  interposition  of  cometary  matter,  comet 
1881,  III.,  presenting  remarkable  cases  of  this  sort.  On  July  8th,  at 
loh.  5om.,  several  small  stars  were  involved  in  this  comet,  one  of 
which  passed  quite  near  the  nucleus  through  the  bright  inner  coma. 
At  that  time  the  comet  was  greatly  disturbed,  its  nucleus  was  con- 
tracting and  enlarging  rapidly,  and  becoming  bright  and  again  faint 
in  an  instant.  Every  time  that  the  nucleus  grew  larger,  the  star  be- 
came invisible,  but  reappeared  the  moment  the  nucleus  was  reduced 
in  size.  This  phenomenon  could  not  be  attributed  to  an  atmospheric 
effect,  since,  while  the  nucleus  was  enlarging,  a  very  small  inner 
nucleus  was  visible  within  the  large  diffused  one,  the  matter  of  which 
had  apparently  spread  over  the  part  of  the  coma  in  which  the  star 
was  involved,  making  it  invisible. 

That  the  mass  of  comets  is  small,  is  proved  by  the  fact  that  they 
have  sometimes  passed  near  the  planets  without  disturbing  them  in 
any  sensible  manner.  Lexell's  comet,  which  in  1770  remained  four 
months  very  near  Jupiter,  did  not  affect  in  the  least  the  orbits,  or 
the  motions  of  its  satellites.  The  same  comet  also  came  within  less 
than  1,500,000  miles  from  the  Earth,  and  on  this  occasion  it  was  cal- 
culated that  its  mass  could  not  have  been  the  3TFVv  Part  of  that  of 


112  THE    TROUVELOT 

our  globe,  since  otherwise  the  perturbations  which  it  would  have 
caused  in  the  elements  of  the  Earth's  orbit  would  have  been  sensi- 
ble. There  was,  however,  no  change.  If  this  comet's  mass  had 
been  equal  to  that  of  our  globe,  the  length  of  our  year  would  have 
been  increased  by  2h.  47m.  The  comet  of  1837  remained  four  days 
within  3,500,000  miles  of  the  Earth,  with  no  sensible  effect. 

It  seems  quite  difficult  to  admit  that  the  denser  part  of  a  comet 
forming  the  nucleus  is  solid,  as  supposed  by  some  physicists,  since 
it  is  so  rapidly  contracted  and  dilated  by  the  solar  forces,  while  the 
comet  is  yet  at  a  too  great  distance  from  the  Sun  to  allow  these 
effects  to  be  attributed  to  solar  heat  alone.  This  part  of  a  comet, 
as  indeed  the  other  parts,  seems  rather  to  be  in  the  gaseous  than  in 
the  solid  state  ;  the  changes  observed  in  the  intensity  of  its  light 
and  in  its  structure  may  be  conceived  as  due  to  some  solar  action 
partaking  of  the  nature  of  electricity. 

It  has  been  a  question  whether  comets  are  self-luminous,  or 
whether  they  simply  reflect  the  solar  light.  When  their  light  is 
analyzed  by  the  spectroscope,  it  is  found  that  the  nucleus  of  a  comet 
generally  gives  a  continuous  spectrum,  while  the  coma  and  tail  give 
a  spectrum  consisting  of  several  bright  diffused  bands.  The  spec- 
trum given  by  the  nucleus  is  rarely  bright  enough  to  allow  the  dark 
lines  of  the  solar  spectrum  to  be  discerned  upon  it  ;  but  such  lines 
were  reported  in  the  spectrum  of  comet  1881,  III.,  a  fact  proving 
that  this  nucleus  at  least  reflected  some  solar  light.  The  nucleus  of 
a  comet  may  be  partly  self-luminous,  and  either  solid,  liquid,  or  com- 
posed of  incandescent  gases  submitted  to  a  great  pressure.  As  to 
the  coma  and  tail,  they  are  evidently  gaseous,  and  partly,  if  not  en- 
tirely, self-luminous,  as  is  proved  by  the  band  spectrum  which  they 
give.  The  position  of  these  bands,  moreover,  indicates  that  the  lu- 
minous gases  of  which  they  are  composed  contain  carbon.  The 
phenomena  of  polarization,  however,  seem  to  prove  that  these  parts 
of  comets  also  reflect  some  solar  light. 

No  theory  so  far  proposed,  to  explain  comets  and  the  strange 
phenomena  they  exhibit,  seems  to  have  been  successful  in  its 
attempts,  and  the  mystery  in  which  these  bodies  have  been  involved 
from  the  beginning  of  their  apparition,  seems  to  be  now  nearly 
as  great  as  ever.  It  has  been  supposed  that  their  tails  have  no 
real  existence,  but  are  due  to  an  optical  illusion.  Prof.  Tyndall  has 
endeavored  to  explain  cometary  phenomena  by  supposing  these 
bodies  to  be  composed  of  vapors  subject  to  decomposition  by  the 
solar  radiations,  and  thus  made  visible,  the  head  and  tail  being  an 
actinic  cloud  due  to  such  decompositions.  According  to  this  view, 


ASTRONOMICAL     DRAWINGS.  118 

the  tails  of  comets  would  not  consist  of  matter  projected  into  space, 
but  simply  of  matter  precipitated  by  the  solar  rays  in  traversing 
the  cometary  nebulosity.  The  endeavor  has  also  been  made  to  ex- 
plain the  various  phenomena  presented  by  comets  by  an  electrical 
action  of  the  Sun  on  the  gases  composing  these  objects.  Theories 
taking  this  as  a  base  seem  to  us  to  be  more  likely  to  lead  to  valuable 
results.  M.  Faye,  who  has  devoted  much  time  and  learning  to  this 
subject,  assumes  a  real  repulsive  force  of  the  Sun,  acting  inversely 
to  the  square  of  the  distance  and  proportionally  to  the  surface,  and 
not  to  the  mass  as  attraction  does.  He  supposes,  however,  that 
this  repulsive  force  is  generated  by  the  solar  heat,  and  not  by  elec- 
tricity. Prof.  Wm.  Harkness  says  that  many  circumstances  seem  to 
indicate  that  the  comets'  tails  are  due,  in  a  great  measure,  to  elec- 
trical phenomena. 

The  fact  that  the  tails  of  comets  are  better  defined  and  brighter 
on  the  forward  side,  associated  with  the  other  fact  that  they  curve 
the  most  when  their  motion  is  most  rapid,  sufficiently  indicates  that 
these  appendages  are  material,  and  that  they  either  encounter  some 
resistance  from  the  medium  in  which  they  move,  or  from  a  solar  re- 
pulsion. The  phenomena  of  condensation  and  extension,  which  I 
have  observed  in  the  comets  of  1874  and  1881,  added  to  the  curious 
behavior  exhibited  by  the  jets  issuing  from  the  nucleus,  seem  to  in- 
dicate the  action  of  electrical  forces  rather  than  of  heat.  The  main 
difficulty  encountered  in  the  framing  of  a  theory  of  comets  consists 
in  explaining  how  so  delicate  and  extended  objects  as  their  tails 
seem  to  be,  can  be  transported  and  whirled  around  the  Sun  at  their 
perihelion  with  such  an  enormous  velocity,  always  keeping  opposite 
to  the  Sun,  and,  as  expressed  by  Sir  John  Herschel,  "  in  defiance  of 
the  law  of  gravitation,  nay,  even  of  the  received  laws  of  motion." 

To  consider  the  direction  of  the  comets'  tails  as  an  indirect  effect 
of  attraction,  seems  out  of  the  question  ;  the  phenomenon  of  repul- 
sion so  plainly  exhibited  by  these  objects  seems  to  point  to  a  posi- 
tive solar  repulsion,  as  alone  competent  to  produce  these  great 
changes.  The  repulsive  action  of  the  Sun  on  comets'  tails  might  be 
conceived,  for  instance,  as  acting  in  a  manner  similar  to  that  of  a 
powerful  current  of  wind  starting  from  the  Sun,  and  constantly  chang- 
ing in  direction,  but  always  keeping  on  a  line  with  the  comet.  Such 
a  current,  acting  on  a  comet's  tail  as  if  it  were  a  pennant,  would 
drive  it  behind  the  nucleus  just  as  observed.  If  it  could  once  be 
ascertained  that  the  great  disturbances  on  comets  correspond  with 
the  magnetic  disturbances  on  our  globe  and  with  the  display  of  the 
auroral  light,  the  electric  nature  of  the  forces  acting  so  strangely  on 


114  THE    TROUVELOT 

the  comets  would  be  substantially  demonstrated.  I  have  shown 
that  some  of  the  great  disturbances  observed  in  the  comets  of  1874 
and  1 88 1  have  coincided  with  auroral  displays,  and  it  will  be  shown 
hereafter  that  similar  displays  have  also  coincided  with  the  passage 
of  meteoric  showers  through  our  atmosphere.  Whether  these  simul- 
taneous phenomena  were  simple  coincidences  having  no  connec- 
tion, or  whether  they  are  the  result  of  a  common  cause,  can  only 
be  ascertained  by  long  continued  future  observations. 


ASTRONOMICAL    DRAWINGS.  115 


SHOOTING-STARS  AND  METEORS. 
PLATE  XII. 

WHILE  contemplating  the  heavens  on  a  clear  moonless  night, 
we  occasionally  witness  the  sudden  blazing  forth  of  a  star-like 
meteor,  which  glides  swiftly  and  silently  across  some  of  the  constel- 
lations, and  as  suddenly  disappears,  leaving  sometimes  along  its 
track  a  phosphorescent  trail,  which  remains  visible  for  a  while  and 
gradually  vanishes.  These  strange  apparitions  of  the  night  are 
called  Falling  or  Shooting-stars. 

There  is  certainly  no  clear  night  throughout  the  year  during 
which  some  of  these  meteors  do  not  make  their  appearance,  but 
their  number  is  quite  variable.  In  ordinary  nights  only  four  or  five 
will  be  observed  by  a  single  person  in  the  course  of  an  hour  ;  but  on 
others  they  are  so  numerous  that  it  becomes  impossible  to  count 
them.  When  the  falling  stars  are  only  a  few  in  number,  and  appear 
scattered  in  the  sky,  they  are  called  Sporadic  Meteors,  and  when 
they  appear  in  great  numbers  they  constitute  Meteoric  Showers  or 
Swarms. 

Probably  there  is  no  celestial  phenomenon  more  impressive  than 
are  these  wonderful  pyrotechnic  displays,  during  which  the  heavens 
seem  to  break  open  and  give  passage  to  fiery  showers,  whose  lumin- 
ous drops  describe  fantastic  hieroglyphics  in  the  sky.  While  observ- 
ing them,  one  can  fully  realize  the  terror  with  which  they  have 
sometimes  filled  beholders,  to  whom  it  seemed  that  the  stability  of 
the  universe  had  come  to  an  end,  and  that  all  the  stars  of  the 
firmament  were  pouring  down  upon  the  Earth  in  deluges  of  fire. 

The  ancients  have  left  record  of  many  great  meteoric  displays, 
and  the  manner  in  which  they  describe  them  sufficiently  indicates 
the  fear  caused  by  these  mysterious  objects.  Among  the  many 
meteoric  showers  recorded  by  ancient  historians  may  be  mentioned 
one  observed  in  Constantinople,  in  the  month  of  November,  472, 
when  all  the  sky  appeared  as  if  on  fire  with  meteors.  In  the  year  599, 


116  THE    TROUVELOT 

meteors  were  seen  on  a  certain  night  flying  in  all  directions  like 
fiery  grasshoppers,  and  giving  much  alarm  to  the  people.  In  March, 
763,  "  the  stars  fell  suddenly,  and  in  such  crowded  number  that 
people  were  much  frightened,  and  believed  the  end  of  the  world  had 
come."  On  April  loth,  1095,  the  stars  fell  in  such  enormous  quan- 
tity from  midnight  till  morning  that  they  were  as  crowded  as  are 
the  hail  stones  during  a  severe  storm. 

In  modern  times  the  fall  of  the  shooting-stars  in  great  number 
has  been  frequently  recorded.  One  of  the  most  remarkable  mete- 
oric showers  of  the  eighteenth  century  occurred  on  the  night  of 
November  I3th,  1799,  and  was  observed  throughout  North  and  South 
America  and  Europe.  On  this  memorable  night  thousands  of  fall- 
ing stars  were  seen  traversing  the  sky  between  midnight  and  morn- 
ing. Humboldt  and  Boupland,  then  traveling  in  South  America, 
observed  the  phenomena  at  Cumana,  between  two  and  five  o'clock 
in  the  morning.  They  saw  an  innumerable  number  of  shooting- 
stars  going  from  north  to  south,  appearing  like  brilliant  fire-works. 
Several  of  these  meteors  left  long  phosphorescent  trails  in  the  sky, 
and  had  nuclei  whose  apparent  diameter,  in  some  cases,  surpassed 
that  of  the  Moon. 

The  shower  of  November  I3th,  1833,  was  still  more  remarkable 
for  the  great  number  of  meteors  which  traversed  the  heavens,  and 
was  visible  over  the  whole  of  North  and  South  America.  On  that 
occasion  the  falling  stars  were  far  too  numerous  to  be  counted,  and 
they  fell  so  thickly  that  Prof.  Olmsted,  of  New  Haven,  who  observed 
them  carefully,  compared  their  number  at  the  moment  of  their  maxi- 
mum fall  to  half  that  of  the  flakes  of  snow  falling  during  a  heavy 
storm.  This  observer  estimated  at  240,000  the  number  of  meteors 
which  must  have  traversed  the  heavens  above  the  horizon  during 
the  seven  hours  while  the  display  was  visible. 

In  the  years  1866,  1867  and  1868,  there  were  also  extraordinary 
meteoric  displays  on  the  night  of  November  I3th.  It  was  on  the 
last  mentioned  date  that  I  had  the  opportunity  to  observe  the  re- 
markable shower  of  shooting-stars  of  which  I  have  attempted  to 
represent  all  the  characteristic  points  in  Plate  XII.  My  observa- 
tions were  begun  a  little  after  midnight,  and  continued  without  in- 
terruption till  sun-rise.  Over  three  thousand  meteors  were  observed 
during  this  interval  of  time  in  the  part  of  the  sky  visible  from  a 
northern  window  of  my  house.  The  maximum  fall  occurred  between 
four  and  five  o'clock,  when  they  appeared  at  a  mean  rate  of  15  in  a 
minute. 

In  general,  the  falling  stars  were  quite  large,  many  being  supe- 


ASTRONOMICAL    DRA  WINGS.  117 

rior  to  Jupiter  in  brightness  and  apparent  size,  while  a  few  even  sur- 
passed Venus,  and  were  so  brilliant  that  opaque  objects  cast  a  strong 
shadow  during  their  flight.  A  great  many  left  behind  them  a  lumin- 
ous train,  which  remained  visible  for  more  or  less  time  after  the 
nucleus  had  vanished.  In  general,  these  meteors  appeared  to  move 
either  in  straight  or  slightly  curved  orbits  ;  but  quite  a  number 
among  them  exhibited  very  extraordinary  motions,  and  followed 
very  complicated  paths,  some  of  which  were  quite  incomprehensible. 

While  some  moved  either  in  wavy  or  zig-zag  lines,  strongly 
accentuated,  others,  after  moving  for  a  time  in  a  straight  line,  grad- 
ually changed  their  course,  curving  upward  or  downward,  thus 
moving  in  a  new  direction.  Several  among  them,  which  were  ap- 
parently moving  in  a  straight  line  with  great  rapidity,  suddenly 
altered  their  course,  starting  at  an  abrupt  angle  in  another  direc- 
tion, with  no  apparent  slackening  in  their  motion.  One  of  them, 
which  was  a  very  conspicuous  object,  was  moving  slowly  in  a 
straight  course,  when  of  a  sudden  it  made  a  sharp  turn  and  con- 
tinued to  travel  in  a  straight  line,  at  an  acute  angle  with  the  first, 
retreating,  and  almost  going  back  towards  the  regions  from  which 
it  originally  came.  As  nearly  all  the  meteors  which  exhibited  these 
extraordinary  motions  left  the  trace  of  their  passage  in  the  sky  by  a 
luminous  trail,  it  was  easily  ascertained  that  these  appearances  were 
not  deceptive.  On  one  occasion  I  noticed  that  the  change  of  direc- 
tion in  the  orbit  corresponded  with  the  brightening  up  of  the  meteor 
thus  disturbed  in  its  progress. 

Among  these  meteors,  some  traveled  very  slowly,  and  a  few 
seemed  to  advance  as  if  by  jerks,  but  in  general  they  moved  very 
rapidly.  One  of  the  meteors  thus  appearing  to  move  by  jerks  left  a 
luminous  trail,  upon  which  the  various  jerks  seemed  to  be  left  im- 
pressed by  a  succession  of  bright  and  faint  spaces  along  the  train. 
Some  of  the  largest  meteors  appeared  to  rotate  upon  an  axis  as 
they  advanced,  and  most  of  these  revolving  meteors,  as  also  a  great 
number  of  the  others,  seemed  to  explode  just  before  they  disap- 
peared, sending  bright  fiery  sparks  of  different  colors  in  all  direc- 
tions, although  no. sound  was  at  any  time  heard.  The  largest  and 
most  brilliant  meteor  observed  on  that  night  appeared  at  5h.  3Om., 
a  little  before  sunrise.  It  was  very  bright,  and  appeared  consider- 
ably larger  than  Venus,  having  quite  a  distinct  disk.  This  meteor 
moved  very  slowly,  leaving  behind  a  large  phosphorescent  trail, 
which  seemed  to  issue  from  the  inside  of  the  nucleus  as  it  advanced. 
For  a  moment  the  train  increased  in  size  and  brightness  close  to 
the  nucleus,  which  then  appeared  as  an  empty  transparent  sphere, 


118  THE    TROUVELOT 

sprinkled  all  over  with  minute  fiery  sparks  ;  the  nucleus  then  sud- 
denly burst  out  into  luminous  particles,  which  immediately  van- 
ished, only  the  luminous  trail  of  considerable  dimensions  being  left. 

Many  of  the  trails  thus  left  by  the  meteors  retained  their  lumin- 
osity for  several  minutes,  and  sometimes  for  over  a  quarter  of  an 
hour.  These  trails  slowly  changed  their  form  and  position;  but  it 
is  perhaps  remarkable  that  almost  all  those  which  I  observed  on 
that  night  assumed  the  same  general  form — that  of  an  open,  irregu- 
lar ring,  or  horse-shoe,  somewhat  resembling  the  letter  C.  This 
ring  form  was  subsequently  transformed  into  an  irregular,  roundish 
cumulus-like  cloud.  The  trail  left  by  a  very  large  meteor,  which 
I  observed  on  the  evening  of  September  5th,  1880,  also  exhibited 
the  same  general  character  of  transformation. 

While  I  was  observing  a  long  brilliant  trail  left  by  a  meteor 
on  the  night  of  November  I3th,  1868,  it  was  suddenly  crossed  by 
another  bright  shooting-star.  The  latter  apparently  went  through 
the  luminous  substance  forming  the  trail,  which  was  suddenly  altered 
in  form,  and  considerably  diminished  in  brightness  simultaneously 
with  this  passage,  although  electrical  action  at  some  distance  might 
perhaps  as  well  explain  the  sudden  change  observed. 

In  the  majority  of  cases  the  meteors  appeared  white;  but  many, 
especially  the  largest,  exhibited  a  variety  of  brilliant  colors,  among 
which  the  red,  blue,  green,  yellow  and  purple  were  the  most  com- 
mon. In  general  the  trails  exhibited  about  the  same  color  as  the 
nucleus,  but  much  fainter,  and  they  were  usually  pervaded  by  a 
greenish  tint.  In  some  instances  the  trails  were  of  quite  a  different 
color  from  the  nucleus. 

The  luminous  cloud  observed  at  5h.  3Om.  on  the  morning  of  No- 
vember I4th,  1868,  after  having  passed  through  the  series  of  trans- 
formations above  described,  remained  visible  for  a  long  while  after 
sunrise,  appearing  then  as  a  small  cirrus  cloud,  exactly  similar  in 
appearance  to  the  hundreds  of  small  cirrus  clouds  then  visible  in  the 
sky,  which  had  probably  the  same  meteoric  origin.  For  over  three 
hours  after  sunrise,  these  cirrus  clouds  remained  visible  in  the  sky, 
moving  all  together  with  the  wind  in  the  high  regions  of  the  atmo- 
sphere. 

Although  Plate  XII.  is  intended  to  represent  all  the  character- 
istics exhibited  by  the  meteors  observed  on  that  night,  every  form 
represented  having  been  obtained  by  direct  observation,  yet  the 
number  is  much  greater  than  it  was  at  any  single  moment  during 
the  particular  shower  of  1868.  As  regards  number,  the  intention 
was  to  give  an  idea  of  a  great  meteoric  shower,  such  as  that  of  1833, 


ASTRONOMICAL    DRA  WINGS.  119 

for  instance.  Although  many  of  the  falling  stars  seem  to  be  close 
to  the  Earth's  surface,  yet  this  is  only  an  effect  of  perspective  due  to 
their  great  distance,  very  few  of  these  meteors  ever  coming  into  the 
lower  regions  of  our  atmosphere  at  all. 

The  phenomena  exhibited  during  other  great  meteoric  showers 
have  been  similar  to  those  presented  by  the  shower  just  described, 
the  only  differences  consisting  in  variations  of  size  and  brightness 
in  the  meteors,  and  also  in  the  trails,  which  sometimes  are  not  so 
numerous  as  they  were  in  1868. 

While  some  shooting-stars  move  so  rapidly  that  they  can  hardly 
be  followed  in  their  orbits,  others  move  so  slowly  that  the  sight  can 
easily  follow  them,  and  even  remark  the  peculiarities  of  their  move- 
ments, some  remaining  visible  for  half  a  minute.  Some  of  the  fall- 
ing stars  move  at  the  rapid  rate  of  100  miles  a  second,  but  others 
only  10  miles  a  second,  and  even  less.  In  general,  they  move  about 
half  as  fast  again  as  the  Earth  in  its  orbit.  The  arcs  described  by 
the  meteors  in  the  sky  are  variable.  While  some  extend  80°  and 
even  100°,  others  are  hardly  half  a  degree  in  length.  While  some 
shooting-stars  are  so  faint  that  they  can  hardly  be  seen  through  the 
largest  telescopes,  others  are  so  large  and  brilliant  that  they  can  be 
seen  in  the  day-time.  In  general,  a  shooting-star  of  average  bright- 
ness resembles  a  star  of  the  third  or  fourth  magnitude. 

Whatever  may  be  the  origin  of  the  shooting-stars,  they  are,  when 
we  see  them,  not  in  the  celestial  spaces,  like  the  planets,  the  comets, 
or  the  stars,  but  in  our  atmosphere,  through  which  they  travel  as 
long  as  they  remain  visible.  The  height  at  which  they  appear  and 
disappear  is  variable,  but  in  general  they  are  about  80  miles  above 
the  surface  of  our  globe  when  they  are  first  seen,  and  at  about  55 
miles  when  they  disappear.  In  many  cases,  however,  they  have 
been  observed  at  greater  elevations,  as  also  at  smaller.  A  meteor 
simultaneously  observed  at  two  different  stations  first  appeared  at 
the  height  of  285  miles,  and  was  last  seen  at  192  miles  above  the 
Earth's  surface;  but  in  rare  cases  the  falling  stars  have  been  seen 
below  a  layer  of  clouds  completely  covering  the  sky.  I  myself  saw 
one  such  shooting-star  a  few  years  since.  The  fact  that  the  meteors 
are  visible  at  so  great  elevations,  proves  that  our  atmosphere  ex- 
tends much  farther  than  was  formerly  supposed,  although  at  these 
great  heights  it  must  be  extremely  rarefied,  and  very  different  from 
what  it  is  in  its  lower  regions. 

There  is  a  remarkable  difference  between  the  sporadic  meteors 
seen  in  the  sky  on  every  night,  and  the  meteoric  showers  observed 
only  at  comparatively  rare  intervals.  While  the  first  appear  from 


120  THE    TROUVELOT 

different  points  in  the  sky  and  travel  in  all  directions,  being  per- 
fectly independent,  the  meteors  of  a  shower  all  come  from  the  same 
point  of  the  heavens,  from  which  they  apparently  diverge  in  all  di- 
rections. This  point  of  divergence  of  the  meteors  is  called  the 
radiant  point  of  the  shower.  Although  the  meteors  seem  to  di- 
verge in  all  directions  from  the  radiant  point,  yet  they  all  move  in 
approximately  parallel  lines,  the  divergence  being  an  effect  of  per- 
spective. 

Whatever  may  be  the  position  of  the  radiant  point  in  the  con- 
stellations, it  remains  as  fixed  in  the  sky  as  the  stars  themselves, 
and  participates  with  them  in  the  apparent  motion  which  they  un- 
dergo by  the  effect  of  the  diurnal  motion,  and  thus  rises  and  sets 
with  the  constellation  to  which  it  belongs.  This  fact  is  sufficient 
to  prove  that  the  orbits  of  these  meteors  are  independent  of  the 
Earth's  motion,  and  that  consequently  they  do  not  originate  in  our 
atmosphere.  It  has  been  shown  by  Encke  that  the  radiant  point 
of  the  meteoric  shower  of  November  I3th  is  precisely  the  point 
towards  which  our  globe  moves  in  space  on  November  I3th;  a  tan- 
gent to  the  Earth's  orbit  would  pass  through  this  radiant  point. 

The  meteoric  showers  are  particularly  remarkable,  not  merely  be- 
cause of  the  large  number  of  meteors  which  are  visible  and  the  fact  that 
they  all  follow  a  common  orbit,  but  chiefly  because  they  have  a  peri- 
odic return,  either  after  an  interval  of  a  year,  or  after  a  lapse  of  sever- 
al years.  At  the  beginning  of  the  present  century  only  two  meteoric 
showers  were  known,  those  of  August  loth  and  of  November  I3th, 
and  their  periodicity  had  not  yet  been  recognized,  although  it  had 
begun  to  be  suspected.  It  was  only  in  1836  that  Quetelet  and  Olbers 
ventured  to  predict  the  reappearance  of  the  November  meteors  in 
the  year  1867.  Having  made  further  investigations,  Prof.  Newton, 
of  Yale  College,  announced  their  return  in  the  year  1866.  In  both 
of  these  years,  as  also  in  1868,  the  meteors  were  very  numerous,  and 
were  observed  in  Europe  and  in  America  on  the  night  of  November 
1 3th.  The  predictions  having  thus  been  fulfilled,  the  periodicity  of 
the  meteors  was  established.  Since  then,  other  periodic  showers 
have  been  recognized,  although  they  are  much  less  important  in 
regard  to  number  than  those  of  August  and  November,  except  that 
of  November  2/th,  which  exhibited  so  brilliant  a  display  in  Europe 
in  1872.  These  successive  appearances  have  established  the  main 
fact  that  meteoric  showers  are  more  or  less  visible  every  year  when 
the  Earth  occupies  certain  positions  in  its  orbit. 

The  meteoric  shower  of  the  loth  of  August  has  its  radiant  point 
situated  in  the  vicinity  of  the  variable  star  Algol,  in  the  constella- 


ASTRONOMICAL    DRAWINGS.  121 

tion  Perseus,  from  which  its  meteors  have  received  the  name  of 
Perseids.  Although  varying  in  splendor,  this  meteoric  swarm  never 
fails  to  make  its  appearance  every  year.  The  Perseids  move  through 
our  atmosphere  at  the  rate  of  37  miles  per  second.  The  shower 
usually  lasts  about  six  hours. 

The  meteoric  shower  of  November  I3th  has  its  radiant  point  sit- 
uated in  the  vicinity  of  the  star  Gamma,  in  the  constellation  Leo, 
from  which  its  meteors  have  been  called  Leonids.  But  while  the 
August  meteors  recur  regularly  every  year,  with  slight  variations, 
the  shower  of  November  does  not  occur  with  the  same  regularity. 
During  several  years  it  is  hardly  noticeable,  and  is  even  totally 
absent,  while  in  other  years  it  is  very  remarkable.  Every  33  years 
an  extraordinary  meteoric  shower  occurs  on  the  I3th  of  November, 
and  the  phenomenon  is  repeated  on  the  two  succeeding  years  at  the 
same  date,  but  with  a  diminution  in  its  splendor  at  each  successive 
return.  The  Leonids  move  in  an  opposite  direction  to  that  of  the 
Earth,  and  travel  in  our  atmosphere  with  an  apparent  velocity  of  45 
miles  per  second,  this  being  about  the  maximum  velocity  observed 
in  falling  stars.  But  when  the  motion  of  our  globe  is  taken  into 
account,  and  a  deduction  is  made  of  the  18  miles  which  it  travels 
per  second,  it  is  found  that  these  meteors  move  at  an  actual  mean 
rate  of  27  miles  a  second. 

In  a  meteoric  shower  the  stars  do  not  fall  uniformly  throughout 
the  night,  there  being  a  time  when  they  appear  in  greater  numbers. 
Usually  it  is  towards  morning,  between  4  and  6  o'clock,  that  the 
maximum  occurs.  The  probable  cause  of  this  phenomenon  will  be 
explained  in  its  place  hereafter. 

The  orbits  of  the  meteoric  showers  are  not  all  approximately  in 
the  same  plane,  like  those  of  the  planets,  but  rather  resemble  those 
of  comets,  and  have  all  possible  inclinations  to  the  ecliptic.  Like 
the  comets,  too,  the  different  meteoric  showers  have  either  direct 
or  retrograde  motion. 

The  shooting-stars  were  formerly  considered  as  atmospheric 
meteors,  caused  by  the  combustion  of  inflammable  gases  generated 
at  the  surface  of  the  Earth,  and  transported  to  the  high  regions  of 
our  atmosphere  by  their  low  specific  gravity.  But  the  considerable 
height  at  which  they  usually  appear,  the  great  velocity  of  their  mo- 
tion, the  common  orbit  followed  by  the  meteors  of  the  same  shower, 
and  the  periodicity  of  their  recurrence,  do  not  permit  us  now  to  en- 
tertain these  ideas,  or  to  doubt  their  cosmical  origin.  But  what  is 
their  -nature  ? 

It  is  now  generally  admitted  that  innumerable  minute  bodies, 


122  THE    TROUVELOT 

moving  in  various  directions  around  the  Sun,  are  scattered  in  the 
interplanetary  spaces  through  which  our  globe  travels.  It  has  been 
supposed  that  congregations  of  such  minute  bodies  form  elliptical 
rings,  within  which  they  are  all  moving  in  close  parallel  orbits  around 
the  Sun.  On  the  supposition  that  such  rings  intersect  the  orbit  of 
the  Earth  at  the  proper  places,  it  was  practicable  to  account  for  the 
shooting-stars  by  the'passage  through  our  atmosphere  of  the  nu- 
merous minute  cosmical  bodies  composing  the  rings,  and  the  Leonid 
and  Perseid  showers  were  so  explained.  But  when  the  elements  of 
the  orbits  of  these  two  last  swarms  came  to  be  better  known,  and 
were  compared  with  those  of  other  celestial  bodies,  it  was  found 
necessary  to  alter  this  theory. 

It  had  for  a  long  while  been  suspected  that  some  kind  of  relation 
existed  between  the  shooting-stars  and  the  comets.  This  idea, 
vaguely  formulated  by  Kepler  more  than  two  centuries  ago,  more 
clearly  expressed  by  Chladni,  and  still  more  by  Mr.  Grey,  before  the 
British  Association,  at  Liverpool,  in  1855,  has  recently  received  a 
brilliant  confirmation  by  the  researches  of  Professor  Schiaparelli, 
Director  of  the  Observatory  of  Milan.  A  thorough  investigation  of 
the  orbits  of  the  August  and  November  meteors  led  Schiaparelli  to 
the  discovery  of  a  remarkable  relation  between  meteoric  and  comet- 
ary  orbits.  By  comparing  the  elements  of  these  meteoric  orbits 
with  those  of  comets,  he  found  a  very  close  resemblance  between  the 
orbit  of  the  August  meteors  and  that  of  the  comet  1862,  III.,  and 
again  between  the  orbit  of  the  November  meteors  and  that  of  Tem- 
pers comet,  1866,  I.  These  resemblances  were  too  striking  to  be 
the  result  of  mere  chance,  and  demonstrated  the  identity  of  these 
cometary  orbits  with  those  of  the  Perseid  and  Leonid  showers. 
In  accordance  with  these  new  facts,  it  is  now  admitted  that  the 
meteoric  showers  result  from  the  passage  of  our  globe  through 
swarms  of  meteoric  particles  following  the  orbits  of  comets,  which 
intersect  the  orbit  of  the  Earth. 

Professor  Schiaparelli  has  attempted  to  show  how  these  meteoric 
swarms  were  originally  scattered  along  the  orbits  of  comets,  by  sup- 
posing these  bodies  to  originate  from  nebulous  masses,  which,  in 
entering  the  sphere  of  attraction  of  the  Sun,  are  gradually  scattered 
along  their  orbits,  and  finally  form  comets  followed  by  long  trails  of 
meteoric  particles.  • 

It  has  been  shown  that  in  approaching  the  Sun  the  comets  be- 
come considerably  elongated,  their  particles  being  disseminated 
over  immense  distances  by  the  solar  repulsion.  It  seems  probable 
that,  owing  to  its  feeble  attractive  power,  the  nucleus  is  incompe- 


ASTRONOMICAL    DRAWINGS.  123 

tent  to  recall  the  scattered  cometary  particles  and  retain  them  in 
its  grasp  when  they  are  relieved  from  the  solar  repulsion,  so  that 
they  remain  free  from  the  nucleus,  although  they  continue  to  move 
along  its  orbit.  It  is  supposable  that  these  cometary  particles  will 
scatter  more  and  more  in  course  of  time.  Forming  at  first  an  elong- 
ated meteoric  cloud,  they  will  finally  spread  along  the  whole  orbit, 
and  thus  form  a  ring  of  meteoric  particles.  Since  our  globe  con- 
stantly moves  in  its  orbit  and  daily  occupies  a  different  position,  it 
follows  that  at  any  point  where  such  a  cometary  orbit  happens  to 
cross  that  of  the  Earth,  our  globe  will  necessarily  encounter  the 
cometary  particles  as  a  shower  of  meteors.  This  encounter  will  take 
place  at  a  certain  time  of  the  year,  either  yearly,  if  they  form  a  con- 
tinuous ring,  or  after  a  succession  of  years,  if  they  simply  form  an 
elongated  cloud.  Such  meteoric  clouds  or  rings  would  not  be  visi- 
ble in  ordinary  circumstances,  even  through  the  largest  telescopes, 
except  on  penetrating  the  upper  regions  of  our  atmosphere,  when 
they  would  appear  as  showers  of  falling  stars.  It  is  supposed  that 
in  penetrating  our  atmosphere,  even  in  its  most  rarefied  regions, 
these  meteors  are  heated  by  the  resistance  offered  by  the  air  to 
their  motion,  first  becoming  luminous  and  then  being  finally  vapor- 
ized and  burnt  before  they  can  reach  the  surface  of  the  Earth. 

The  orbit  of  the  comet  of  1862,  III.,  which  so  closely  corresponds 
with  that  of  the  Perseid  meteors,  is  much  more  extended  than  that 
of  Tempel's  comet  corresponding  with  that  of  the  Leonids.  While 
the  first  extends  far  beyond  the  orbit  of  Neptune,  the  latter  only  goes 
a  little  beyond  that  of  Uranus.  The  former  orbit  makes  a  consider- 
able angle  with  the  plane  of  the  Earth's  orbit,  but  the  latter  is  much 
nearer  to  parallelism  with  it.  The  period  of  revolution  of  the  first  is 
108  years,  and  that  of  the  last  about  33^  years. 

From  the  fact  that  the  Perseid  shower  occurs  yearly  on  the  loth 
of  August,  when  the  Earth  crosses  the  orbit  of  the  comet  of  1862, 
III.,  it  is  supposed  that  the  cometary  particles  producing  this  shower 
are  disseminated  along  the  whole  orbit,  and  form  a  ring  encircling 
the  Sun  and  Earth.  To  explain  the  yearly  variations  in  the  number 
of  the  shooting-stars  observed,  these  particles  are  supposed  to  be  un- 
equally distributed  over  the  orbit,  being  more  crowded  at  one  place 
than  they  are  at  another.  In  order  to  explain  the  meteoric  shower 
of  Leonids,  which  appears  in  all  its  splendor  every  33  years,  and 
then  with  diminished  intensity  for  two  successive  years,  after  which 
it  is  without  importance,  it  is  supposed  that  the  cometary  particles 
of  the  comet  of  1866,  I.,  have  not  as  yet  spread  all  along  the  orbit, 
a  sufficient  time  not  having  been  allowed,  but  form  an  elongated 


124  THE    TROUVELOT 

meteoric  cloud,  more  dense  in  its  front  than  in  its  rear  part.  From 
these  considerations  it  has  been  supposed  also  that  the  comet  of 
1866,  I.,  is  of  a  more  recent  date  than  that  of  1862,  III.  While 
Tempel's  comet  makes  its  revolution  around  the  Sun  in  about  33 
years,  this  meteoric  cloud,  which  has  the  same  period  and  returns  to 
the  same  point  of  its  orbit  every  33  years,  encounters  our  globe  for 
three  successive  years.  The  first  year  we  are  passing  through  its 
densest  parts,  and  the  two  following  years  in  less  and  less  crowded 
parts,  from  which  result  the  observed  phenomena.  An  idea  of  the 
extent  of  this  meteoric  cloud  may  be  formed  from  the  fact  that,  with 
its  cometary  velocity  of  motion,  it  takes  this  cloud  three  years  at 
least  to  cross  the  Earth's  orbit.  From  recent  researches  it  would 
appear  that  the  Leonid  cloud  is  not  single,  but  that  at  least  two 
others  of  smaller  importance  exist,  and  have  periods  of  33^  years. 

Biela's  comet,  which  was  divided  into  two  parts  in  1846,  is 
another  of  the  few  comets  whose  orbit  approaches  that  of  the 
Earth.  Possessing  this  knowledge,  and  knowing  then  the  close  con- 
nection existing  between  meteors  and  comets,  astronomers  supposed 
that  there  were  sufficient  reasons  to  expect  a  meteoric  shower  when 
this  comet  was  passing  near  the  Earth.  They  consequently  expect- 
ed a  meteoric  display  in  1872,  when  our  globe  was  to  cross  its  orbit. 
Their  anticipation  was  plainly  fulfilled,  and  on  the  night  of  Novem- 
ber 27th,  1872,  a  splendid  meteoric  display,  having  its  radiant  point 
in  the  constellation  Andromeda,  was  observed  in  Europe,  and  also 
in  America,  but  the  meteors  seen  here  were  not  so  numerous  as  in  Eu- 
rope. Other  meteoric  showers  of  less  importance,  such  as  that  of 
April  2Oth,  for  instance,  have  also  been  identified  with  cometary  or- 
bits, so  that  now  no  doubt  seems  to  remain  as  to  the  identity  of  com- 
etary particles  and  shooting-stars. 

The  fact  that  the  maximum  number  of  meteors  is  always  ob- 
served in  the  morning  hours,  supports  the  hypothesis  of  the  cosmic 
origin  of  the  shooting-stars,  since  the  regions  of  the  Earth  where  it 
is  morning  are  precisely  those  fronting  the  regions  towards  which 
our  globe  is  moving  in  space,  and  accordingly  encounter  more  di- 
rectly the  meteors  moving  in  their  orbit.  The  greater  abundance 
of  falling  stars  at  that  time  may  thus  be  accounted  for. 

The  number  of  meteors  penetrating  our  atmosphere  must  be 
very  great;  there  is  not  an  hour  and  probably  not  a  minute  during 
which  none  fall.  From  various  considerations,  some  astronomers 
have  estimated  at  from  65,000,000,000  to  146,000,000,000  the  total 
number  of  shooting-stars  yearly  penetrating  in  our  atmosphere. 
The  actual  number  is  undoubtedly  great,  yet  the  fact  that  the 


ASTRONOMICAL    DRAWINGS.  125 

meteors  are  rarely  seen  through  the  telescope  while  employed  in 
observing  various  celestial  objects,  does  not  indicate  that  they  are 
so  numerous  as  these  figures  imply.  It  is  only  occasionally  that 
one  is  seen  traversing  the  field  of  the  instrument.  Even  when  the 
sky  is  observed  with  a  low  power  eye-piece  for  several  hours  in  suc- 
cession, many  nights  may  pass  without  disclosing  one,  although 
an  observer,  sweeping  the  sky  more  freely  with  the  naked  eye,  may 
often  perceive  four  or  five  during  an  ordinary  night. 

About  the  true  nature  of  these  bodies  nothing  is  known  with 
certainty.  From  spectrum  analysis  it  seems  to  be  established  that 
most  of  them  contain  sodium  and  magnesium,  while  a  few  indicate 
the  presence  of  strontium  and  iron,  and  in  some  rare  cases  there  are 
traces  of  coal-gas.  Some  of  the  nuclei  give  a  continuous  spectrum, 
and  others  a  spectrum  of  lines.  The  trail  always  gives  a  spectrum 
of  bright  lines  which  indicates  its  gaseous  state.  The  traces  of 
coal-gas  rarely  seen  in  meteors  are,  however,  of  great  importance, 
as  it  identifies  them  more  closely  with  the  comets,  which  generally 
show  a  similar  spectrum.  The  continuous  spectra  exhibited  by 
some  nuclei  would  indicate  that  they  are  incandescent  and  either 
solid  or  liquid;  but  it  is  difficult  to  conclude  from  their  spectra  what 
is  their  true  nature,  since  we  do  not  know  exactly  what  part  the 
terrestrial  atmosphere  may  play  in  producing  the  results. 

The  mass  of  the  shooting-stars  is  not  known  with  certainty,  but 
the  fact  that  during  great  meteoric  showers,  none  are  seen  to  reach 
the  surface  of  the  Earth,  all  being  consumed  in  a  few  seconds,  suf- 
ficiently indicates  that  it  must  be  very  small.  It  has  been  calculated 
that  those  equal  to  Venus  in  apparent  size  and  brilliancy  may  weigh 
several  pounds,  while  the  faint  ones  would  weigh  only  a  few  grains. 

If  the  shooting-stars  have  even  such  a  mass  as  that  here  attrib- 
uted to  some  of  them,  the  extraordinary  motions  which  I  have  de- 
scribed above  seem  to  be  unaccountable.  The  change  of  direction 
of  a  heavy  mass  moving  swiftly  cannot  be  sudden.  The  semi-cir- 
cular, the  wavy  and  the  angular  orbits  observed  could  not  be  de- 
scribed, it  would  seem,  by  such  a  mass  animated  with  a  great  ve- 
locity. Although  the  meteors  are  said  to  be  ignited  by  the  trans- 
formation of  part  of  their  progressive  motion  into  molecular  motion, 
yet  it  is  not  observed  that  the  velocity  of  the  falling  stars  diminishes 
when  they  are  about  to  disappear.  The  luminous  trails  they  leave 
in  the  atmosphere  do  not  appear  to  be  endowed  with  any  motion, 
but  remain  for  a  time  in  their  original  positions.  These  facts  are 
apparently  opposed  to  the  hypothesis  that  such  meteors  have  any 
appreciable  mass.  The  extraordinary  motions  exhibited  by  some 


126  THE     TROUVELOT 

meteors  seem  to  indicate  that  some  unsuspected  force  resides  in 
these  bodies,  and  causes  them  to  deviate  from  the  laws  of  ordinary 
motion. 

Although  it  is  very  probable  that  the  ordinary  shooting-stars 
have  no  appreciable  mass,  yet  it  is  known  that  very  heavy  meteoric 
masses  sometimes  fall  at  the  surface  of  the  Earth.  Such  falls  are 
generally  preceded  by  the  sudden  apparition  in  the  sky  of  a  large, 
and  usually  very  brilliant  fire-ball,  which  traverses  the  air  at  a  great 
speed,  sometimes  leaving  behind  it  a  luminous  trail,  after  which  it 
explodes  with  a  loud  sound,  and  heavy  fiery  meteoric  fragments, 
diverging  in  all  directions,  fall  at  the  surface  of  the  Earth.  The 
name  of  Aerolites  or  Meteor  elites  is  given  to  these  ponderous  frag- 
ments. As  these  meteors,  before  they  explode  and  fall  to  the  ground, 
have  many  points  of  resemblance  with  the  shooting-stars,  they  are 
generally  supposed  to  be  connected  with  them,  and  to  have  a  simi- 
lar cometary  origin.  The  fact  that  the  aerolites  differ  widely  from 
each  other  in  constitution,  and  are  all  composed  of  substances  found 
on  the  Earth,  associated  with  other  facts  given  below,  would  rather 
seem  to  indicate  a  terrestrial  than  a  celestial  origin. 

If  the  aerolites  belong  to  the  same  class  of  bodies  as  the  falling 
stars,  differing  from  them  only  in  size  and  mass,  it  is  difficult  to  see 
why  so  very  few  should  fall  upon  the  Earth  during  the  great  meteoric 
showers,  when  thousands  of  shooting-stars  traverse  our  atmosphere. 
In  Prof.  Kirkwood's  "Meteoric  Astronomy"  are  given  catalogues 
of  all  the  falls  of  aerolites  and  fire-balls  which  have  been  observed 
at  the  time  of  the  periodic  meteoric  showers  of  the  roth  of  August 
and  the  I3th  of  November,  during  a  period  of  221  years  for  the 
Perseids,  or  August  showers,  and  of  318  years  for  the  Leonids,  or 
November  showers.  During  221  years,  10  falls  of  aerolites  have  been 
witnessed  simultaneously  with  the  fall  of  the  Perseids;  while  during 
318  years,  only  4  such  falls  have  been  recorded  as  having  occurred 
at  the  time  of  the  Leonid  shower.  If  there  is  any  close  connection 
between  the  shooting-stars  and  the  aerolites,  we  should  expect  to 
find  a  maximum  in  their  fall  at  the  time  of  the  great  meteoric  .dis- 
plays. So  far,  no  maxima  or  minima  have  yet  been  discovered  in 
the  fall  of  aerolites;  they  do  not  seem,  like  meteoric  showers,  to  be 
governed  by  a  law  of  periodicity. 

A  very  remarkable  peculiarity  of  the  aerolites  is  that  they  seem 
to  have  a  tendency  to  fall  in  certain  regions.  Such  are  the  southern 
part  of  France,  the  north  of  Italy,  Hindostan,  the  central  states  of 
North  America,  and  Mexico  and  Brazil.  There  is  a  curious  contrast 
existing  between  the  quick  cometary  motion  of  the  aerolites  before 


ASTRONOMICAL    DRAWINGS.  127 

their  explosion,  and  the  comparatively  slow  motion  of  their  frag- 
ments as  they  reach  the  Earth;  motion  which  seems  to  be  no  greater 
than  that  corresponding  to  their  natural  fall  impeded  by  the  resist- 
ance of  the  air.  In  general,  their  penetration  into  the  soil  upon 
which  they  fall  does  not  at  all  correspond  to  the  great  velocity  with 
which  they  move  in  the  atmosphere.  The  fragmentary  structures 
of  the  aerolites,  their  identity  of  substance  with  that  of  our  globe, 
their  great  resemblance  to  the  volcanic  minerals  of  the  Earth,  and 
the  fractures  and  faults  which  some  of  them  exhibit,  do  not  corre- 
spond at  all  with  the  idea  that  they  are  cometary  particles  fallen  on 
the  Earth.  As  far  as  their  structure  and  appearance  is  concerned, 
they  seem  rather  to  be  a  volcanic  product  of  the  interior  of  the 
Earth  than  parts  of  disintegrated  comets.  It  must  be  admitted  that 
their  identity  with  the  shooting-stars  is  far  from  established,  and 
that  they  are  still  involved  in  mystery. 

The  so-called  meteoric  dust  gathered  at  sea  and  on  high  mount- 
ains may  have  various  origins,  and  may  be  partly  furnished  by  vol- 
canic dust  carried  to  great  distances  in  the  atmosphere. 

Since  millions  of  shooting-stars  penetrate  our  atmosphere  every 
year  and  remain  in  it,  becoming  definitively  a  part  of  the  Earth,  it 
follows  that,  no  matter  how  small  may  be  the  quantity  of  matter  of 
which  they  are  composed,  they  must  gradually  increase  the  volume 
and  mass  of  our  globe,  although  the  increase  may  be  exceedingly 
slow.  Supposing  every  one  of  the  shooting-stars  penetrating  our 
atmosphere  to  contain  one  cubic  millimeter  of  matter,  it  has  been 
calculated  that  it  would  take  nearly  35,000  years  to  make  a  deposit 
one  centimeter  in  thickness  all  over  the  surface  of  our  globe.  In- 
significant as  this  may  appear,  it  is  probable  that  the  quantity  of 
matter  of  meteoric  origin  which  is  added  to  our  globe  is  much  less 
than  has  just  been  supposed. 


128  THE    TROUVELOT 


THE    MILKY-WAY    OE    GALAXY. 
PLATE    XIII. 

DURING  clear  nights,  when  the  Moon  is  below  the  horizon,  the 
starry  vault  is  greatly  adorned  by  an  immense  belt  of  soft  white 
light,  spanning  the  heavens  from  one  point  of  the  horizon  to  the 
opposite  point,  and  girdling  the  celestial  sphere  in  its  delicate  folds. 
Every  one  is  familiar  with  this  remarkable  celestial  object,  called 
the  Milky-way  or  Galaxy. 

Seen  with  the  naked  eye,  the  Galaxy  appears  as  an  irregular, 
narrow,  nebulous  belt,  apparently  composed  of  cloud-like  luminous 
masses  of  different  forms  and  sizes,  separated  by  comparatively  dark 
intervals.  These  cloud-like  masses  vary  much  in  luminous  intensity, 
and  while  some  among  them  are  very  bright  and  conspicuous,  others 
are  so  faint  that  they  are  hard  to  recognize.  In  general,  the  bright- 
est parts  of  the  Milky- way  are  situated  along  the  middle  of  its  belt, 
while  its  borders,  which  are  usually  very  faint,  gradually  vanish  in 
the  sky.  Some  parts  of  the  Galaxy,  however,  show  very  little  of 
the  cloudy  structure  so  characteristic  of  other  parts,  being  almost 
uniform  throughout,  except  towards  the  borders,  which  are  always 
fainter.  These  parts  showing  greater  uniformity  are  also  the  faintest. 

Such  is  the  general  appearance  of  the  Milky-way  on  ordinary 
nights,  but  on  rare  occasions,  when  the  atmosphere  is  particularly 
pure,  it  presents  one  of  the  grandest  sights  that  can  be  imagined. 
At  such  favorable  moments  I  have  seen  the  Galaxy  gleaming  with 
light,  and  appearing  as  if  composed  of  star-dust  or  of  precious  stones. 
The  strange  belt  then  appeared  all  mottled  over  and  fleecy,  its 
large  cloud-like  masses  being  subdivided  into  numerous  small, 
irregular  cloudlets  of  great  brilliancy,  which  appeared  projected  upon 
a  soft  luminous  background. 

The  width  of  the  Galaxy  is  far  from  being  uniform;  while  in  some 
places  it  is  only  4°  or  5°,  in  others  it  is  15°  and  even  more.  In  some 
places  it  appears  wavy  in  outline,  at  others  quite  straight;  then  it 


ASTRONOMICAL    DRAWINGS.  129 

contracts,  to  expand  a  few  degrees  distant  ;  while  at  other  places 
it  sends  off  branches  and  loops,  varying  in  form,  size  and  direction, 
some  of  which  are  quite  prominent,  while  others  are  very  faint. 

Although  very  irregular  in  form,  the  general  appearance  of  the 
galactic  belt  is  that  of  a  regular  curve  occupying  one  of  the  great  cir- 
cles of  the  celestial  sphere.  The  Milky-way  completely  encircles 
the  heavens,  but,  of  course,  only  one-half  is  visible  at  any  one 
moment,  since  our  globe  prevents  the,  other  half  from  being  seen. 
If,  for  a  moment,  we  imagine  ourselves  left  in  space,  our  globe  hav- 
ing vanished  from  under  our  feet,  we  should  then  see  the  whole 
Galaxy  forming  a  continuous  belt  in  the  heavens,  at  the  centre  of 
which  we  should  apparently  be  situated. 

While  only  one-half  of  the  galactic  belt  can  be  seen  at  once 
from  any  point  on  the  Earth,  yet,  according  to  the  position  of  the 
observer,  a  larger  or  smaller  portion  of  the  whole  can  be  seen  at 
different  times.  In  high  northern  or  southern  latitudes  but  little 
more  than  half  can  be  seen  even  by  continuous  observations;  but  as 
we  approach  the  equatorial  regions,  more  and  more  of  it  becomes 
visible,  until  the  whole  may  be  seen  at  different  hours  and  seasons. 
In  the  latitudes  of  the  northern  states,  about  two-thirds  of  the 
Galaxy  is  visible,  the  rest  remaining  hidden  below  the  horizon  ; 
but  from  the  southern  states  very  nearly  the  whole  can  be  seen. 
The  half  of  the  Milky-way  visible  at  any  one  time  from  any  lati- 
tude on  the  Earth  never  entirely  sets  below  the  horizon,  although 
in  some  places  it  may  be  so  near  the  horizon  as  to  be  rendered  in- 
visible by  vapors.  In  the  latitude  of  Cambridge,  when  in  its  lowest 
position,  the  summit  of  its  arc  is  still  about  12°  or  15°  above  the 
northern  horizon.  The  great  circle  of  the  celestial  sphere,  occupied 
by  the  galactic  belt,  is  inclined  at  an  angle  of  about  63°  to  the  celes- 
tial equator,  and  intersects  this  great  circle  on  one  side  in  the  con- 
stellation Monoceros  in  6h.  47m.,  and  on  the  opposite  side  in  the 
constellations  Aquila  and  Ophiuchus  in  i8h.  4/m.  of  right  ascension; 
so  that  its  northern  pole  is  situated  in  the  constellation  Coma  Be- 
renices in  R.  A.  I2h.  47m.,  declination  N.  27°,  and  the  southern  in 
the  constellation  Cetus  in  R.  A.  o  h.  47m.,  declination  S.  27.° 

According  to  the  seasons  and  to  the  hours  of  the  night  at  which 
it  is  observed,  the  galactic  arch  presents  different  inclinations  in  the 
sky.  Owing  to  its  inclination  to  the  equator  of  the  celestial  sphere, 
its  opposite  parts  exhibit  opposite  inclinations  when  they  pass  the 
meridian  of  a  place.  That  part  of  the  Galaxy  which  is  represented 
on  I^late  XIII.,  and  which  intersects  the  celestial  equator  in  the  con- 
stellation Aquila,  is  inclined  to  the  left  or  towards  the  east,  when 


130  THE    TROUVELOT 

it  is  on  the  meridian;  while  the  opposite  part,  situated  in  Monocerosr 
is  inclined  to  the  right,  or  towards  the  west,  when  it  reaches  the 
meridian.  The  former  passes  the  meridian  in  the  evening  in  the 
summer  and  autumn  months;  the  latter,  in  the  winter  and  spring 
months. 

By  beginning  at  its  northernmost  part,  represented  at  the  upper 
part  of  Plate  XIII. ,  situated  in  "the  chair"  of  the  constellation 
Cassiopeia,  and  descending  southwardly,  and  continuing  in  the  same 
direction  until  the  whole  circle  is  completed,  the  course  of  the  Milky- 
way  through  the  constellations  may  be  briefly  described  as  follows: 
From  Cassiopeia's  chair,  the  Galaxy,  forming  two  streams,  descends 
south,  passing  partly  through  Lacerta  on  the  left,  and  Cepheus  on 
the  right;  at  this  last  point  it  approaches  nearest  to  the  polar  star. 
Then  it  enters  Cygnus,  where  it  becomes  very  complicated  and 
bright,  and  where  several  large  cloudy  masses  are  seen  terminat- 
ing its  left  branch,  which  passes  to  the  right,  near  the  bright  star 
Deneb,  the  leader  of  this  constellation.  Below  Deneb,  the  Galaxy 
is  apparently  disconnected  and  separated  from  the  northern  part  by 
a  narrow,  irregular  dark  gap.  From  this  rupture,  the  Milky-way 
divides  into  two  great  streams  separated  by  an  irregular,  dark  rift. 
An  immense  branch  extends  to  the  right,  which,  after  having  formed 
an  important  luminous  mass  between  the  stars  f  and  /9,  continues 
its  southward  progress  through  parts  of  Lyra,  Vulpecula,  Hercules, 
Aquila  and  Ophiuchus,  where  it  gradually  terminates  a  few  degrees 
south  of  the  equator.  The  main  stream  on  the  left,  after  having 
formed  a  bright  mass  around  e  Cygni,  passes  through  Vulpecula 
and  then  Aquila,  where  it  crosses  the  equinoctial  just  below  the 
star  7],  after  having  involved  in  its  nebulosity  the  bright  star  Altair, 
the  leader  of  Aquila.  In  the  southern  hemisphere  the  Galaxy  be- 
comes very  complicated  and  forms  a  succession  of  very  bright, 
irregular  masses,  the  upper  one  being  in  Scutum  Sobieskii,  while 
the  others  are  respectively  situated  in  Sagittarius  and  in  Scorpio; 
the  last,  just  a  little  above  our  horizon,  being  always  considerably 
dimmed  by  vapors.  From  Scutum  Sobieskii,  the  Galaxy  expands 
considerably  on  the  right,  and  sends  a  branch  into  Scorpio,  in  which 
the  fiery  red  star  Antares  is  somewhat  involved. 

Continuing  its  course  below  our  horizon,  the  Milky-way  enters 
Ara  and  Norma,  and  then,  passing  partly  through  Circinus,  Centaurus 
and  Musca,  it  reaches  the  Southern  Cross,  after  having  been  divided 
by  the  large  dark  pear-shaped  spot  known  to  navigators  as  the 
"  Coal-Sack."  In  Ara  and  Crux  the  Milky-way  attains  its  maxi- 
mum of  brightness,  which  there  surpasses  its  brightest  parts  in  Cyg- 


ASTRONOMICAL    DRA  WINGS.  131 

nus.  In  Musca,  it  makes  its  nearest  approach  to  the  south  pole 
of  the  heavens.  It  then  enters  Carina  and  Vela,  where  it  spreads 
out  like  a  fan,  and  terminates  in  this  last  constellation,  before  reach- 
ing /,  being  once  more  interrupted  by  a  dark  and  very  irregular 
gap,  on  a  line  with  the  two  star  sj-  and  X.  It  is  noteworthy  that  this 
second  rupture  of  continuity  of  the  Galaxy  in  Vela  is  very  nearly 
opposite,  or  at  about  180°  from  the  break  near  Deneb  in  Cygnus. 

Continuing  its  course  on  the  other  side  of  the  break,  the  Milky- 
way  again  spreads  out  into  the  shape  of  a  fan,  grows  narrower  in 
entering  Puppis,  where  it  is  longitudinally  divided  by  darkish  chan- 
nels. It  then  passes  above  our  southern  horizon,  becoming  visible 
to  us,  passing  through  part  of  Canis  Major,  where  its  border  just 
grazes  the  brilliant  star  Sirius.  But  from  Puppis  it  gradually  dimin- 
ishes in  brightness  and  complication,  becoming  faint  and  uniform. 
It  enters  Monoceros  and  Orion,  where  it  again  crosses  the  equator  a 
little  above  d,  the  northernmost  of  the  three  bright  stars  in  the 
belt  of  Orion.  Continuing  its  northward  course  it  passes  through 
Gemini,  extending  as  far  as  Castor  and  Pollux,  and  then  entering 
Auriga,  where  it  begins  to  increase  in  brightness  and  in  complica- 
tion of  structure.  It  passes  partly  through  Camelopardus  and  into 
Perseus,  where  an  important  branch  proceeds  from  its  southern  bor- 
der. 

This  branch  beginning  near  the  star  6r  advances  towards  the 
celebrated  variable  star  Algol,  around  which  it  is  quite  bright  and 
complicated.  Continuing  its  course  in  the  same  direction,  the  branch 
rapidly  loses  its  brightness,  becoming  very  faint  a  little  below  Algol, 
and  passing  through  f  Persei,  it  enters  Taurus,  leaving  the  Pleiades 
on  its  extreme  southern  margin;  and  after  having  passed  through  ef 
where  it  branches  off,  it  rapidly  curves  towards  the  main  stream, 
which  it  joins  near  £  Tauri,  thus  forming  an  immense  loop.  The 
ramification  projecting  near  e  Tauri  involves  in  its  nebulosity  the 
ruddy  star  Aldebaran  and  the  scattered  group  of  the  Hyades.  It 
then  advances  towards  the  three  bright  stars  d,  £  and  f  of  the  belt 
of  Orion,  which,  together  with  the  sextuple  star  6  Orionis,  are  in- 
volved in  its  faint  nebulosity,  and  joins  the  main  stream  on  the 
equinoctial,  having  thus  formed  a  second  loop,  whose  interior  part 
is  comparatively  free  from  nebulosity,  and  contains  the  fine  stars 
Betelgeuse  and  Bellatrix. 

That  portion  of  the  main  galactic  stream  which  is  comprised  be- 
tween the  star  Deneb  in  Cygnus,  and  Capella  in  Auriga,  is  divided 
longitudinally  by  a  very  irregular,  narrow,  darkish  cleft,  compara- 
tively devoid  of  nebulosity,  which,  however,  is  interrupted  at  some 


132  THE    TROUVELOT 

points.  This  dark  gap  sends  short  branches  north  and  south,  the 
most  important  of  which  are  situated  near  f  Cephei  and  /9  Cas- 
siopeiae.  Another  branch  runs  from  7-  beyond  e  of  the  constellation 
last  mentioned.  The  main  stream  of  the  Galaxy  after  leaving  Per- 
seus, enters  Cassiopeia,  and  sending  short  branches  into  Andromeda, 
it  completes  its  immense  circle  in  Cassiopeia's  chair,  where  this  de- 
scription was  begun. 

When  examined  through  the  telescope,  the  appearance  of  the 
Milky- way  completely  changes,  and  its  nebulous  light  is  resolved  into 
an  immense  number  of  stars,  too  faint  to  be  individually  seen  with 
the  naked  eye.  When  Galileo  first  directed  the  telescope  to  the 
galactic  belt,  its  nebulous,  cloud-like  masses  were  at  once  resolved 
into  stars,  even  by  the  feeble  magnifying  power  of  his  instrument. 
When,  much  later,  Sir  William  Herschel  undertook  his  celebrated 
star-gaugings  of  the  Galaxy,  millions  of  stars  blazed  out  in  his  pow- 
erful telescopes.  The  stars  composing  this  great  nebulous  belt  are 
so  numerous  that  it  is  impossible  to  arrive  at  any  definite  idea  as  to 
their  number.  From  his  soundings  Herschel  estimated  at  116,000 
the  number  of  stars  which,  on  one  occasion,  passed  through  the  field 
of  his  telescope  in  15  minutes,  by  the  simple  effect  of  the  diurnal 
motion  of  the  heavens  ;  and  on  another  occasion,  a  number  estimated 
at  250,000  crossed  the  field  in  41  minutes.  In  a  space  of  5°,  com- 
prised between  ft  and  7-  Cygni,  shown  on  Plate  XIII.,  he  found  no 
less  than  331,000  stars.  Prof.  Struve  has  estimated  at  20,500,000  the 
number  of  stars  seen  in  the  Milky- way  through  the  twenty-foot  teles- 
cope employed  by  Herschel  in  his  star-gaugings.  Great  as  this  num- 
ber may  seem,  it  is  yet  far  below  the  truth;  as  the  great  modern 
telescopes,  according  to  Professor  Newcomb,  would  very  probably 
double  the  number  of  stars  seen  through  Herschel's  largest  teles- 
cope, and  detect  from  thirty  to  fifty  millions  of  stars  in  the  Milky- 
way. 

Although  the  telescope  resolves  the  Galaxy  into  millions  of 
stars,  yet  the  largest  instruments  fail  to  penetrate  its  immense  depths. 
The  forty-foot  telescope  of  Herschel,  and  even  the  giant  telescope 
of  Lord  Rosse,  have  failed  to  resolve  the  Milky-way  entirely  into 
stars,  the  most  distant  ones  appearing  in  them  as  nebulosities  upon 
which  the  nearer  stars  are  seen  projected,  the  galactic  stratum 
being  unfathomable  by  the  largest  telescopes  yet  made. 

The  stars  composing  the  Milky-way  are  very  unevenly  distrib- 
uted, as  might  easily  be  supposed  from  the  cloud-like  appearance 
of  this  belt.  In  some  regions  they  are  loosely  scattered,  forming 
long  rows  or  streams  of  various  figures,  while  in  others  they  congre- 


ASTRONOMICAL     DRA  WINGS.  133 

gate  into  star  groups  and  clusters  having  all  imaginable  forms, 
some  being  compressed  into  very  dense  globular  masses.  The  in- 
tervals left  between  the  clustering  masses  are  poorer  in  stars,  and 
indeed  some  of  them  are  even  totally  devoid  of  stars  or  nebulosity. 
Such  are  the  great  and  small  "  coal-sacks  "  in  the  southern  Galaxy. 
I  have  myself  detected  such  a  dark  space  devoid  of  stars  and  nebu- 
losity in  one  of  the  brightest  parts  of  the  Milky-way,  in  the  con- 
stellation Sagittarius,  in  about  i/h.  45m.  right  ascension,  and  27° 
35'  south  declination.  It  is  a  small  miniature  coal-sack  or  opening 
in  the  Galaxy,  through  which  the  sight  penetrates  beyond  this  great 
assemblage  of  stars.  Close  to  this,  is  another  narrow  opening  near 
a  small,  loose  cluster. 

Although  lacking  the  optical  resources  which  now  enable  us  to 
recognize  the  structure  of  the  Milky-way,  some  of  the  ancient  phi- 
losophers had  succeeded  tolerably  well  in  their  speculations  re- 
garding its  nature.  It  was  the  opinion  of  Democritus,  Pythagoras 
and  Manilius,  that  the  Galaxy  was  nothing  else  but  a  vast  and  con- 
fused assemblage  of  stars,  whose  faint  light  was  the  true  cause  of 
its  milky  appearance. 

Before  the  invention  of  the  telescope,  no  well-founded  theory  in 
regard  to  the  structure  of  the  Milky-way  could,  of  course,  be  at- 
tempted. Although  Kepler  entertained  different  ideas  in  regard  to 
the  structure  of  this  great  belt  from  those  now  generally  admitted, 
yet  in  them  may  be  found  the  starting  point  of  the  modern  concep- 
tion of  the  structure  of  the  Galaxy  and  of  the  visible  universe.  In 
the  view  of  this  great  mind,  the  Milky-way,  with  all  its  stars,  formed 
a  vast  system,  the  centre  of  which,  and  of  the  universe,  was  occupied 
by  our  Sun.  Kepler  reasoned  that  the  place  of  the  Sun  must  be  near 
the  centre  of  the  galactic  belt,  from  the  fact  this  last  object  appears 
very  nearly  as  a  great  circle  of  the  celestial  sphere,  and  that  its 
luminous  intensity  is  about  the  same  in  all  its  parts. 

Half  a  century  later,  another  attempt  to  explain  the  Milky-way 
was  made  by  Wright,  of  Durham,  who  rejected  the  idea  of  an  acci- 
dental and  confused  distribution  of  the  stars  as  inconsistent  with  the 
appearance  of  the  Galaxy,  and  regarded  them  as  arranged  along  a 
fundamental  plane  corresponding  to  that  of  the  Milky-way.  These 
ideas  which  were  subsequently  developed  and  enlarged  by  Kant, 
and  then  by  Lambert,  constitute  what  is  now  known  as  Kant's 
theory.  According  to  this  theory,  the  stars  composing  the  Galaxy 
are  conceived  as  being  uniformly  arranged  between  two  flat  planes 
of  considerable  extension,  but  which  are  comparatively  near  to- 
gether, the  $un  occupying  a  place  not  very  far  from  the  centre  of 


134  THE    TROUVELOT 

this  immense  starry  stratum.  As  we  view  this  system  crosswise 
through  its  thinnest  parts,  the  stars  composing  it  appear  scattered 
and  comparatively  few  in  number,  but  when  we  view  it  lengthwise, 
through  its  most  extended  parts,  they  appear  condensed  and  ex- 
tremely numerous,  thus  giving  the  impression  of  a  luminous  belt 
encircling  the  heavens.  In  the  conception  of  Kant,  each  star  was  a 
sun,  forming  the  centre  of  a  planetary  system.  These  systems  are  not 
independent,  but  are  kept  together  by  the  bonds  of  universal  gravi- 
tation. The  Galaxy  itself  is  one  of  these  great  systems,  its  princi- 
pal plane  being  the  equivalent  of  the  zodiac  in  our  planetary  system, 
while  a  preponderant  body,  which  might  be  Sirius,  is  the  equivalent 
of  our  Sun,  and  keeps  the  galactic  system  together.  In  the  universe 
there  are  other  galaxies,  but  as  they  are  too  distant  to  be  resolved 
into  stars,  they  appear  as  elliptical  nebulae.  Such  are,  in  brief,  the 
grand  speculations  of  Kant  and  Lambert  on  the  Milky-way,  and  the 
structure  of  the  universe. 

Kant's  theory  rested  more  on  conjectures  than  on  observed 
facts,  and  needed  therefore  the  sanction  of  direct  observations  to 
be  established  on  a  firm  basis.  With  this  view,  Sir  William  Her- 
schel  investigated  the  subject,  by  a  long  and  laborious  series  of  ob- 
servations. His  plan,  which  was  that  of  "  star-gauging,"  consisted 
in  counting  all  the  stars  visible  in  his  twenty-foot  telescope,  com- 
prised in  a  wide  belt  cutting  the  Galaxy  at  right  angles,  and  ex- 
tending from  one  of  its  sides  to  the  opposite  one,  thus  embracing 
180°  of  the  celestial  sphere.  In  this  belt  he  executed  3,400  tele- 
scopic star-gaugings  of  a  quarter  of  a  degree  each,  from  which  he 
obtained  683  mean  gaugings  giving  the  stellar  density  of  the  cor- 
responding regions. 

The  general  result  derived  from  this  immense  labor  was  that 
the  stars  are  fewest  in  regions  the  most  distant  from  the  galactic 
belt;  while  from  these  regions,  which  correspond  to  the  pole  of  the 
Galaxy,  they  gradually  increase  in  number  in  approaching  the 
Milky-way.  The  star  density  was  found  to  be  extremely  variable, 
and  while  some  of  the  telescopic  gaugings  detected  either  no  star  at 
all,  or  only  one  or  two,  other  gaugings  gave  500  stars  and  even  more. 
The  average  number  of  stars  in  a  field  of  view  of  his  telescope,, 
obtained  for  the  six  zones,  each  of  15°,  into  which  Herschel  divided 
up  the  portion  of  his  observing  belt,  extending  from  the  Galaxy 
to  its  pole,  is  as  follows:  In  the  first  zone,  commencing  at  90^ 
from  the  galactic  belt  and  extending  towards  it,  4  stars  per  tele- 
scopic field  were  found;  5  in  the  second;  8  in  the  third;  14  in  the 
fourth;  24  in  the  fifth  and  53  in  the  sixth,  which  terminated  in  the 


ASTRONOMICAL     DRAWINGS.  135 

Galaxy  itself.  Very  nearly  similar  results  were  afterwards  found  by 
Sir  John  Herschel,  for  corresponding  regions  in  the  southern  hemi- 
sphere. 

From  these  studies,  Herschel  concluded  that  the  stellar  sys- 
tem is  of  the  general  form  supposed  by  the  Kantian  theory,  and 
that  its  diameter  must  be  five  times  as  extended  in  the  direction  of 
the  galactic  plane,  as  it  is  in  a  direction  perpendicular  to  it.  To 
explain  the  great  branch  sent  out  by  the  Galaxy  in  Cygnus,  he  sup- 
posed a  great  cleft  dividing  the  system  edgewise,  about  half  way 
from  its  circumference  to  its  centre.  From  suppositions  founded  on 
the  apparent  magnitude  and  arrangement  of  stars,  he  estimated  that 
it  would  take  light  about  7,000  years  to  reach  us  from  the  extremi- 
ties of  the  Galaxy,  and  therefore  14,000  years  to  travel  across  the 
system,  from  one  border  to  the  opposite  one. 

But  Herschel's  theory  concerning  the  Milky-way  rested  on  the 
erroneous  assumption  that  the  stars  are  uniformly  distributed  in 
space,  and  also  that  his  telescopes  penetrated  through  the  entire 
depth  of  the  Galaxy.  Further  study  showed  him  that  his  telescope 
of  twenty  feet,  and  even  his  great  forty-foot  telescope,  which  was 
estimated  to  penetrate  to  a  distance  2,300  times  that  of  stars  of  the 
first  magnitude,  failed  to  resolve  some  parts  of  the  Galaxy  into 
stars.  Meanwhile,  the  structure  of  the  Milky-way  being  better 
known,  the  irregular  condensation  of  its  stars  became  apparent, 
while  the  mutual  relation  existing  between  binary  and  multiple  sys- 
tems of  stars,  as  also  between  the  stars  which  form  clusters,  was 
recognized,  as  showing  evidence  of  closer  association  between  cer- 
tain groups  of  stars  than  between  the  stars  in  general.  Herschel's 
system,  which  rested  on  the  assumption  of  the  uniform  distribution 
of  the  stars  in  space,  and  on  the  supposition  that  the  telescopes  used 
for  his  gauges  penetrated  through  the  greater  depths  of  the  Galaxy, 
being  thus  found  to  contradict  the  facts,  was  gradually  abandoned 
by  its  author,  who  adopted  another  method  of  estimating  the  rela- 
tive distances  of  the  stars  observed  in  his  gaugings. 

This  method,  founded  on  photometric  principles,  consisted  in 
judging  the  penetrating  power  of  his  telescope  by  the  brightness  of 
the  stars,  and  not,  as  formerly,  by  the  number  which  they  brought 
into  view.  He  then  studied  by  this  new  method  the  structure  of  the 
Milky-way  and  the  probable  distance  of  the  clustering  masses  of 
which  it  is  formed,  concluding  that  the  portion  of  the  Galaxy  travers- 
ing the  constellation  Orion  is  the  nearest  to  us.  This  last  result 
seems  indicated  by  the  fact  that  this  portion  of  the  Milky-way  is  the 
faintest  and  the  most  uniform  of  all  the  galactic  belt. 


136 


THE    TROUVELOT 


More  recently  Otto  Struve  investigated  the  same  subject,  and 
arrived  at  very  nearly  similar  conclusions,  which  may  be  briefly  stated 
as  follows  :     The  galactic  system  is  composed  of  a  countless  num- 
ber  of  stars,  spreading   out    on    all    sides  along  a  very  extended 
plane.      These  stars,  which  are  very  unevenly  distributed,  show  a 
decided  tendency  to  cluster  together  into  individual  groups  of  differ- 
ent   sizes   and    forms,    separated    by  comparatively  vacant  spaces. 
This  layer  where  the  stars  congregate  in  such  vast  numbers  may 
be  conceived  as  a  very  irregular  flat  disk,  sending  many  branches 
in  various  directions,  and  having  a  diameter  eight  or  ten  times  its 
thickness.     The  size  of  this  starry  disk  cannot  be  determined,  since 
it  is  unfathomable  in  some  directions,  even  when  examined   with 
the  largest  telescopes.     The  Sun,  with  its  attending  planets,  is  in- 
volved in  this  immense  congregation  of  suns,  of  which  it  forms  but 
a  small  particle,  occupying  a  position  at   some  distance  from  the 
principal  plane  of  the  Galaxy.     According  to  Struve,  this  distance 
is  approximately  equal  to  208,000  times  the  radius  of  the  Earth's 
orbit.      The  Milky-way  is  mainly  composed  of  star-clusters,  two- 
thirds,  perhaps,  of  the  whole  number  visible  in  the  heavens  being  in- 
volved in  this  great  belt.     In  conclusion,  our  Sun  is  only  one  of  the 
individual  stars  which  constitute  the  galactic  system,  and  each  of 
these  stars  itself  is  a  sun  similar  to  our  Sun.     These  individual  suns 
are  not  independent,  but  are  associated  in  groups  varying  in  num- 
ber from  a  few  to  several  thousands,  the  Galaxy  itself  being  noth- 
ing but  an    immense  aggregation    of  such    clusters,    whose  whole 
number  of  individual  suns  probably  ranges  between  thirty  and  fifty 
millions.    In  this  vast  system  our  globe  is  so  insignificant  that  it  can- 
not even  be  regarded  as  one  of  its  members.      According  to  Dr. 
Gould,  there  are  reasons  to  believe  that  our  Sun  is  a  member  of 
a  small,  flattened,  bifid   cluster,  composed  of  more  than  400  stars, 
ranging  between  the  first  and  seventh    magnitude,  its  position  in 
this  small  system  being  eccentric,  but  not  very  far  from  the  galactic 
plane. 

The  study  of  the  Milky-way,  of  which  Plate  XIII.  is  only  a  part, 
was  undertaken  to  answer  a  friendly  appeal  made  by  Mr.  A.  Marth, 
in  the  Monthly  Notices  of  the  Royal  Astronomical  Society,  in  1872. 
I  take  pleasure  in  offering  him  my  thanks  for  the  suggestion,  and  for 
the  facility  afforded  me  in  this  study  by  his  "  List  of  Co-ordinates  of 
Stars  within  and  near  the  Milky-way,"  which  was  published  with  it. 


ASTRONOMICAL    DRAWINGS.  137 


THE  STAR-CLUSTERS. 
PLATE  XIV. 

IT  is  a  well-known  fact  that  the  stars  visible  to  the  naked  eye 
are  very  unequally  distributed  in  the  heavens,  and  that  while  they 
are  loosely  scattered  in  some  regions,  in  others  they  are  compara- 
tively numerous,  sometimes  forming  groups  in  which  they  appear 
quite  close  together. 

In  our  northern  sky  are  found  a  few  such  agglomerations  of  stars, 
which  are  familiar  objects  to  all  observers  of  celestial  objects.  In 
the  constellation  Coma  Berenices,  the  stars  are  small,  but  quite 
condensed,  and  form  a  loosely  scattered,  faint  group.  In  Taurus, 
the  Hyades  and  the  Pleiades,  visible  during  our  winter  nights,  are 
conspicuous  and  familiar  objects  which  cannot  fail  to  be  recognized. 
In  the  last  group,  six  stars  may  be  easily  detected  by  ordinary  eyes 
on  any  clear  night,  but  more  can  sometimes  be  seen;  on  rare  occa- 
sions, when  the  sky  was  especially  favorable,  I  have  detected  eleven 
clearly  and  suspected  several  others.  The  six  stars  ordinarily  visi- 
ble, are  in  order  of  decreasing  brightness,  as  follows  :  Alcyone, 
Electra,  Atlas,  Maia,  Taygeta  and  Merope.  Glimpses  of  Celano  and 
Pleione  are  sometimes  obtained. 

When  the  sky  is  examined  with  some  attention  on  any  clear, 
moonless  night,  small,  hazy,  luminous  patches,  having  a  cometary 
aspect,  are  visible  here  and  there  to  the  naked  eye.  In  the  constel- 
lation Cancer  is  found  one  of  the  most  conspicuous,  called  Praesepe, 
which  forms  a  small  triangle  with  the  two  stars  7-  and  d.  In  Perseus, 
and  involved  in  the  Milky-way,  is  found  another  luminous  cloud, 
situated  in  the  sword-handle,  and  almost  in  a  line  with  the  two 
stars  f  and  d  of  Cassiopeia's  Chair.  In  the  constellation  Hercules, 
another  nebulous  mass  of  light,  but  fainter,  is  also  visible  between 
the  stars  y  and  £,  where  it  appears  as  a  faint  comet,  in  the  depths 
of  space.  In  Ophiuchus  and  Monoceros  are  likewise  found  hazy,  lu- 
minous patches.  In  the  southern  sky,  several  such  objects  are  also 
visible  to  the  naked  eye,  being  found  in  Sagittarius,  in  Canis  Major 


138  THE     TROUVELOT 

and  in  Puppis;  but  the  most  conspicuous  are  those  in  Centaurus  and 
Toucan.  That  in  Centaurus  involves  the  star  to  in  its  pale  diffused 
nebulosity,  and  that  in  Toucan  is  involved  in  the  lesser  Magellanic 
cloud. 

When  the  telescope  is  directed  to  these  nebulous  objects,  their 
hazy,  ill-defined  aspect  disappears,  and  they  are  found  to  consist  of 
individual  stars  of  different  magnitudes,  which  being  more  or  less 
closely  grouped  together,  apparently  form  a  system  of  their  own. 
These  groups,  which  are  so  well  adapted  to  give  us  an  insight  into 
the  structure  and  the  vastness  of  the  stellar  universe,  are  called 
Star-clusters. 

Star-clusters  are  found  of  all  degrees  of  aggregation,  and  while 
in  some  of  them,  such  as  in  the  Pleiades,  in  Praesepe  and  in  Perseus, 
the  stars  are  so  loosely  scattered  that  an  opera  glass,  and  even  the 
naked  eye,  will  resolve  them;  in  others,  such  as  in  those  situated  in 
Hercules,  Aquarius,  Toucan  and  Centaurus,  they  are  so  greatly 
compressed  that  even  in  the  largest  telescopes  they  appear  as  a 
confused  mass  of  blazing  dust,  in  which  comparatively  few  individual 
stars  can  be  distinctly  recognized.  Although  only  about  a  dozen 
Star-clusters  can  be  seen  in  the  sky  with  the  naked  eye,  yet  nearly 
eleven  hundred  such  objects  visible  through  the  telescope,  have 
been  catalogued  by  astronomers. 

The  stars  composing  the  different  clusters  visible  in  the  heavens 
vary  greatly  in  number,  and  while  in  some  clusters  there  are  only  a 
few,  in  others  they  are  so  numerous  and  crowded  that  it  would  be 
idle  to  try  to  count  them,  their  number  amounting  to  several  thou- 
sands. It  has  been  calculated  by  Herschel  that  some  clusters  are 
so  closely  condensed,  that  in  an  area  not  more  than  TV  part  of  that 
covered  by  the  Moon,  at  least  5,000  stars  are  agglomerated. 

When  the  group  in  the  Pleiades  is  seen  through  the  telescope  it 
appears  more  important  than  it  does  to  the  naked  eye,  and  several 
hundreds  of  stars  are  found  in  it.  In  a  study  of  Tempel's  nebula, 
which  is  involved  in  the  Pleiades,  I  have  mapped  out  250  stars, 
mostly  comprised  within  this  nebula,  with  the  telescope  of  6j^  inch- 
es aperture,  which  I  have  used  for  this  study. 

As  a  type  of  a  loose,  coarse  cluster,  that  in  Perseus  is  one  of  the 
finest  of  its  class.  It  appears  to  the  naked  eye  as  a  single  object, 
but  in  the  telescope  it  has  two  centres  of  condensation,  around 
which  cluster  a  great  number  of  bright  stars,  forming  various  curves 
and  festoons  of  great  beauty.  Among  its  components  are  found 
several  yellow  and  red  stars,  which  give  a  most  beautiful  contrast 
of  colors  in  this  gorgeous  and  sparkling  region.  In  a  study  which  I 


ASTRONOMICAL     DRAWINGS.  139 

have  made  of  this  twin  cluster,   I  have  mapped  out  664  stars  be- 
longing to  it,  among  which  are  two  yellow  and  five  red  stars. 

While  some  clusters,  like  those  just  described,  are  very  easily 
resolvable  into  stars  with  the  smallest  instruments,  others  yield  with 
the  greatest  difficulty,  even  to  the  largest  telescopes,  in  which  their 
starry  nature  is  barely  suspected.  Owing  to  this  peculiarity,  star- 
clusters  are  usually  divided  into  two  principal  classes.  In  the  first 
class  are  comprised  all  the  clusters  which  have  been  plainly  resolved 
into  stars,  and  in  the  second  all  those  which,  although  not  plainly 
resolvable  with  the  largest  instruments  now  at  our  disposal,  show  a 
decided  tendency  to  resolvability,  and  convey  the  impression  that 
an  increase  of  power  in  telescopes  is  the  only  thing  needed  to  re- 
solve them  into  stars.  Of  course  this  classification,  which  depends 
on  the  power  of  telescopes  to  decide  the  nature  of  these  objects,  is 
arbitrary,  and  a  classification  based  on  spectrum  analysis  is  now  sub- 
stituted for  it. 

The  star-clusters  are  also  divided  into  globular  and  irregular 
clusters,  according  to  their  general  form  and  appearance.  The 
globular  clusters,  which  are  the  most  numerous,  are  usually  well- 
defined  objects,  more  or  less  circular  in  their  general  outlines.  The 
rapid  increase  of  brightness  towards  their  centres,  where  the  stars 
composing  them  are  greatly  condensed,  readily  conveys  the  im- 
pression that  the  general  form  of  these  sparkling  masses  is  globular. 
The  irregular  clusters  are  not  so  rich  in  stars  as  the  former.  Usually 
their  stars  are  less  condensed  towards  the  centre,  and  are,  for  the 
most  part,  so  loosely  and  irregularly  distributed,  that  it  is  impossi- 
ble to  recognize  the  outlines  of  these  clusters  or  to  decide  where 
they  terminate.  The  globular  clusters  are  usually  quite  easily  re- 
solvable into  stars,  either  partly  or  wholly,  although  some  among 
them  do  not  show  the  least  traces  of  resolvability,  even  in  the 
largest  instruments.  This  may  result  from  different  causes,  and 
may  be  attributed  either  to  the  minuteness  of  their  components  or 
to  their  great  distance  from  the  Earth,  many  star-clusters  being  at 
such  immense  distances  that  they  are  beyond  our  means  of  measure- 
ment. 

As  has  been  shown  in  the  preceding  section,  the  star-clusters 
are  found  in  great  number  in  the  Galaxy;  indeed,  it  is  in  this  region 
and  in  its  vicinity  that  the  greater  portion  of  them  are  found.  In 
other  regions,  with  the  exception  of  the  Magellanic  clouds,  where 
they  are  found  in  great  number  and  in  every  stage  of  resolution,  the 
clusters  are  few  and  scattered. 

The  star-cluster  in  the  constellation  Hercules,  designated  as  No. 


140 


THE     TROUVELOT 


4,230  in  Sir  J.  Herschel's  catalogue,  and  which  is  represented  on 
Plate  XIV.,  is  one  of  the  brightest  and  most  condensed  in  the 
northern  hemisphere,  although  it  is  not  so  extended  as  several 
others,  its  angular  diameter  being  only  7'  or  8'.  This  object,  which 
was  discovered  by  Halley  in  1714,  is  one  of  the  most  beautiful  of  its 
class  in  the  heavens.  According  to  Herschel,  it  is  composed  of 
thousands  of  stars  between  the  tenth  and  fifteenth  magnitudes. 
Undoubtedly  the  stars  composing  this  group  are  very  numerous,  al- 
though those  which  can  be  distinctly  seen  as  individual  stars,  and 
whose  position  can  be  determined,  are  not  so  many  as  a  superficial 
look  at  the  object  would  lead  us  to  suppose.  From  a  long  study  of 
this  cluster,  which  I  have  made  with  instruments  of  various  aper- 
tures, I  have  not  been  able  to  identify  more  stars  than  are  repre- 
sented on  the  plate,  although  the  nebulosity  of  which  this  object 
mainly  consists,  and  especially  the  region  situated  towards  its  cen- 
tre, appeared  at  times  granular  and  blazing  with  countless  points 
of  light,  too  faint  and  too  flickering  to  be  individually  recognized. 
Towards  its  centre  there  is  quite  an  extended  region,  whose  lumin- 
ous intensity  is  very  great,  and  which  irresistibly  conveys  the  im- 
pression of  the  globular  structure  of  this  cluster.  Besides  several 
outlying  appendages,  formed  by  its  nebulosity,  the  larger  stars  re- 
cognized in  this  cluster  are  scattered  and  distributed  in  such  a  way 
that  they  form  various  branches,  corresponding  with  those  formed  by 
the  irresolvable  nebulosity.  At  least  six  or  seven  of  these  branches 
and  wings  are  recognized,  some  of  which  are  curved  and  bent  in 
various  ways,  thus  giving  this  object  a  distant  resemblance  to 
some  crustacean  forms.  Although  I  have  looked  for  it  with  care,  I 
have  failed  to  recognize  the  spiral  structure  attributed  to  this  object 
by  several  observers.  Among  the  six  appendages  which  I  have 
recognized,  some  are  slightly  curved;  but  their  curves  are  sometimes 
in  opposite  directions,  and  two  branches  of  the  upper  portion  make 
so  short  a  bend  that  they  resemble  a  claw  rather  than  a  spiral 
wing.  The  spectrum  of  this  cluster,  like  that  of  many  objects  of  its 
class,  is  continuous,  with  the  red  end  deficient. 

A  little  to  the  north-east  of  this  object  is  found  the  cluster  No. 
4,294,  which,  although  smaller  and  less  bright  than  the  preceding,  is 
still  quite  interesting.  It  appears  as  a  distinctly  globular  cluster 
without  wings,  and  much  condensed  towards  its  centre.  The  stars 
individually  recognized  in  it,  although  less  bright  than  those  of  the 
other  cluster,  are  so  very  curiously  distributed  in  curved  lines  that 
they  give  a  peculiar  appearance  to  this  condensed  region. 

A  little  to   the  north   of  7-   Centauri   may  be  found  the  great 


ASTRONOMICAL     DRAWINGS.  141 

to  Centauri  cluster,  No.  3,531,  already  referred  to  above.  This 
magnificent  object,  which  appears  as  a  blazing  globe  20'  in  diame- 
ter, is,  according  to  Herschel,  the  richest  in  the  sky,  and  is  resolved 
into  a  countless  number  of  stars  from  the  twelfth  to  the  fifteenth 
magnitude,  which  are  greatly  compressed  towards  the  centre.  The 
larger  stars  are  so  arranged  as  to  form  a  sort  of  net-work,  with  two- 
dark  spaces  in  the  middle. 

The  great  globular  cluster  No.  52,  involved  in  the  lesser  Magel- 
lanic  cloud,  in  the  constellation  Toucan,  is  a  beautiful  and  remark- 
able object.  It  is  composed  of  three  distinct,  concentric  layers  of 
stars,  varying  in  brightness  and  in  degree  of  condensation  in  each 
layer.  The  central  mass,  which  is  the  largest  and  most  brilliant,  is; 
composed  of  an  immense  number  of  stars  greatly  compressed,  whose, 
reddish  color  gives  to  this  blazing  circle  a  splendid  appearance. 
Around  the  sparkling  centre  is  a  broad  circle,  composed  of  less 
compressed  stars,  this  circle  being  itself  involved  in  another  cir- 
cular layer,  where  the  stars  are  fainter  and  more  scattered  and 
gradually  fade  away. 

Many  other  great  globular  clusters  are  found  in  various  parts  of 
the  heavens,  among  which  may  be  mentioned  the  cluster  No.  4,678,. 
in  Aquarius.  This  object  is  composed  of  several  thousand  stars  of 
the  fifteenth  magnitude,  greatly  condensed  towards  the  centre,  and, 
as  remarked  by  Sir  J.  Herschel,  since  the  brightness  of  this  cluster 
does  not  exceed  that  of  a  star  of  the  sixth  magnitude,  it  follows  that 
in  this  case  several  thousand  stars  of  the  fifteenth  magnitude  equal 
only  a  star  of  the  sixth  magnitude.  In  the  constellation  Serpens 
the  globular  clusters  No.  4,083  and  No.  4,1 18  are  both  conspicuous 
objects,  also  No.  4,687  in  Capricornus.  In  Scutum  Sobieskii  the 
cluster  No.  4,437  is  one  of  the  most  remarkable  of  this  region.  The 
stars  composing  it,  which  are  quite  large  and  easily  made  out  separ- 
ately, form  various  figures,  in  which  the  square  predominates. 

Among  the  loose  irregular  clusters,  some  are  very  remarkable  for 
the  curious  arrangement  of  their  stars.  In  the  constellation  Gemini 
the  cluster  No.  1,360,  which  is  visible  to  the  naked  eye,  is  a  magni- 
ficent object  seen  through  the  telescope,  in  which  its  sparkling  stars 
form  curves  and  festoons  of  great  elegance.  The  cluster  No.  1,467, 
of  the  same  constellation,  is  remarkable  for  its  triangular  form.  In 
the  constellation  Ara  the  cluster  No.  4,233,  composed  of  loosely 
scattered  stars,  forming  various  lines  and  curves,  is  enclosed  orr 
three  sides  by  nearly  straight  single  lines  of  stars.  In  Scorpio  the 
cluster  No.  4,224  is  still  more  curious,  being  composed  of  a  continu- 
ous ring  of  loosely  scattered  stars,  inside  of  which  is  a  round,  loose- 


142  THE    TROUVELOT 

cluster,  which  is  divided  into  four  parts  by  a  dark  cross-shaped  gap, 
in  which  no  stars  are  visible. 

Among  the  1,034  objects  which  are  now  classified  as  clusters 
more  or  less  resolvable,  565  have  been  absolutely  resolved  into 
stars,  and  469  have  been  only  partly  resolved,  but  are  considered 
as  belonging  to  this  class  of  objects.  In  Sir  J.  Herschel's  catalogue 
there  are  102  clusters  which  are  considered  as  being  globular;  among 
them  30  have  been  positively  resolved  into  stars. 

The  agglomeration  of  thousands  of  stars  into  a  globular  cluster 
cannot  be  conceived,  of  course,  to  be  simply  the  result  of  chance. 
This  globular  form  seems  clearly  to  indicate  the  existence  of  some 
bond  of  union,  some  general  attractive  force  acting  between  the 
different  members  of  these  systems,  which  keeps  them  together, 
and  condenses  them  towards  the  centre.  Herschel  regards  the 
loose,  irregular  clusters  as  systems  in  a  less  advanced  stage  of  con- 
densation, but  gradually  concentrating  by  their  mutual  attraction 
into  the  globular  form.  Although  the  stars  of  some  globular  clus- 
ters appear  very  close  together,  they  are  not  necessarily  so,  and 
may  be  separated  by  great  intervals  of  space.  It  has  been  shown 
that  the  clusters  are  agglomeration  of  suns,  and  that  our  Sun  itself 
is  a  member  of  a  cluster  composed  of  several  hundreds  of  suns, 
although,  from  our  point  of  observation,  these  do  not  seem  very  close 
together.  So  far  as  known,  the  nearest  star  to  us  is  a  Centauri,  but 
its  distance  from  the  Earth  equals  221,000  times  the  distance  of  the 
Sun  from  our  globe,  a  distance  which  cannot  be  traversed  by  light 
in  less  than  three  years  and  five  months.  It  seems  very  probable 
that  if  the  suns  composing  the  globular  clusters  appear  so  near 
together,  it  is  because,  in  the  first  place,  they  are  at  immense  dis- 
tances from  us,  and  in  the  second,  because  they  appear  nearly  in 
a  line  with  other  suns,  which  are  at  a  still  greater  distance  from 
us,  and  on  which  they  accordingly  are  nearly  projected.  If  one 
should  imagine  himself  placed  at  the  centre  of  the  cluster  in  Her- 
cules, for  instance,  the  stars,  which  from  our  Earth  seems  to  be  so 
closely  grouped,  would  then  quite  likely  appear  very  loosely  scat- 
tered around  him  in  the  sky,  and  would  resemble  the  fixed  stars  as 
seen  from  our  terrestrial  station. 

Judging  by  their  loose  and  irregular  distribution,  the  easily  re- 
solvable clusters  would  appear,  in  general,  to  be  the  nearer  to  us. 
It  is  probable  that  the  globular  clusters  do  not  possess,  to  a  very 
great  degree,  the  regular  form  which  they  ordinarily  present  to 
us.  It  seems  rather  more  natural  to  infer  that  they  are  irregular, 
and  composed  of  many  wings  and  branches,  such  as  are  observed 


ASTRONOMICAL    DRA  WINGS.  143 

in  the  cluster  in  Hercules;  but  as  these  appendages  would  neces- 
sarily be  much  poorer  in  stars  than  the  central  portions,  they  would 
be  likely  to  become  invisible  at  a  great  distance,  and  therefore  the 
object  would  appear  more  or  less  globular;  the  globular  form  being 
simply  given  by  the  close  grouping  of  the  stars  in  the  central  por- 
tion. It  would  seem,  then,  that  in  general,  the  most  loosely  scat- 
tered and  irregular  clusters  are  the  nearest  to  us,  while  the  smallest 
globular  clusters  and  those  resolvable  with  most  difficulty  are  the 
most  distant. 

In  accordance  with  the  theory  that  the  clusters  are  composed  of 
stars,  the  spectrum  of  these  objects  is  in  general  continuous;  al- 
though, in  many  cases,  the  red  end  of  the  spectrum  is  either  very 
faint  or  altogether  wanting.  Many  objects  presenting  in  a  very 
high  degree  the  principal  characteristics  exhibited  by  the  true  star- 
clusters,  namely,  a  circular  or  oval  mass,  whose  luminous  intensity 
is  greatly  condensed  toward  the  centre,  have  not  yielded,  however, 
to  the  resolving  power  of  the  largest  telescopes,  although  their  con- 
tinuous spectrum  is  in  close  agreement  with  their  general  resem- 
blance to  the  star-clusters.  Although  such  objects  may  remain 
irresolvable  forever,  yet  it  is  highly  probable  that  they  do  not  mate- 
rially differ  from  the  resolvable  and  partly  resolvable  clusters,  except 
by  their  enormous  distance  from  us,  which  probably  reaches  the  ex- 
treme boundary  of  our  visible  universe. 


144  THE    TROUVELOT 


THE  NEBULAE. 
PLATE  XV. 

Besides  the  foggy,  luminous  patches  which  have  just  been  de- 
scribed, a  few  hazy  spots  of  a  different  kind  are  also  visible  to  the 
naked  eye  on  any  clear,  moonless  night.  These  objects  mainly 
differ  from  the  former  in  this  particular,  that  when  viewed  through 
the  largest  telescopes  in  existence  they  are  not  resolved  into  stars, 
but  still  retain  the  same  cloudy  appearance  which  they  present  to 
the  unassisted  eye.  On  account  of  the  misty  and  vaporous  appear- 
ance which  they  exhibit,  these  objects  have  been  called  Nebula. 

Of  the  26  nebulous  objects  visible  to  the  naked  eye  in  the  whole 
heavens,  19  belong  to  the  class  of  star-clusters,  and  7  to  the  class  of 
nebulae.  Among  the  most  conspicuous  nebulae  visible  to  the  unas- 
sisted eye,  are  those  in  the  constellations  Argo  Navis,  Andromeda 
and  Orion. 

Besides  the  seven  nebulae  visible  to  the  naked  eye,  a  great  num- 
ber of  similar  objects  are  visible  through  the  telescope.  In  Sir  John 
Herschel's  catalogue  of  nebulae  and  clusters,  are  found  4,053  irre- 
solvable nebulae,  and  with  every  increase  of  the  aperture  of  tele- 
scopes, new  nebulae,  invisible  in  smaller  instruments,  are  found. 
Notwithstanding  their  irresolvability  it  is  probable,  however,  that 
many  among  them  have  a  stellar  structure,  which  their  immense 
distance  prevents  us  from  recognizing,  and  are  not  therefore  true 
nebulae.  The  giant  telescope  of  Lord  Rosse  has  shown  nebulae  so 
remote  that  it  has  been  estimated  that  it  takes  their  light  30  mil- 
lion years  to  reach  the  Earth. 

The  nebulae  are  very  far  from  being  uniformly  distributed  in 
space.  In  some  regions  they  are  rare,  while  in  others  they  are  nu- 
merous and  crowded  together,  forming  many  small,  irregular  groups, 
differing  in  size  and  in  richness  of  aggregation.  The  grouping  of 
the  nebulae  does  not  occur  at  random  in  any  part  of  the  heavens,  as 
might  naturally  be  supposed,  but,  on  the  contrary,  it  is  chiefly  con- 


ASTRONOMICAL    DRAWINGS.  145 

fined  to  certain  regions.  Outside  of  these  regions  nebulae  are  rare 
and  are  separated  from  each  other  by  immense  intervals;  so  that 
these  isolated  objects  appear  as  if  they  were  lost  wanderers  from 
the  great  nebulous  systems. 

The  regions  where  the  nebulae  congregate  in  great  number  are 
very  extensive,  and  in  a  general  view  there  are  two  vast  systems  of 
nebular  agglomeration,  occupying  almost  opposite  points  of  the 
heavens,  whose  centres  are  not  very  distant  from  the  poles  of 
the  Milky-way.  In  the  northern  hemisphere,  the  nebulous  system 
is  much  richer  and  more  condensed  than  in  the  southern  hemi- 
sphere. The  northern  nebulae  are  principally  contained  in  the  con- 
stellations Ursa  Minor  and  Major,  in  Draco,  Canes  Venatici,  Bootes, 
Leo  Major  and  Minor,  Coma  Berenices,  and  Virgo.  In  this  region, 
which  occupies  about  y§  of  the  whole  surface  of  the  heavens,  %  of 
the  known  nebulae  are  assembled.  The  southern  nebulae  are  more 
evenly  distributed  and  less  numerous,  with  the  exception  of  two 
comparatively  small,  but  very  remarkable  centres  of  condensation 
which,  together  with  many  star-clusters,  constitute  the  Magellanic 
clouds. 

These  two  vast  nebular  groups  are  by  no  means  regular  in  out- 
line, and  send  various  branches  toward  each  other.  They  are  sep- 
arated by  a  wide  and  very  irregular  belt,  comparatively  free  from 
nebulae,  which  encircles  the  celestial  sphere,  and  whose  medial  line 
approximately  coincides  with  that  of  the  galactic  belt.  The  Milky- 
way,  so  rich  in  star-clusters,  is  very  barren  in  nebulae;  but  it  is  a 
very  remarkable  fact,  nevertheless,  that  almost  all  the  brightest, 
largest,  and  most  complicated  nebulae  of  the  heavens  are  situated 
either  within  it,  or  in  its  immediate  vicinity.  Such  are  the  great 
nebulae  in  Orion  and  Andromeda;  the  nebula  of  £  Orionis;  the  Ring 
nebula  in  Lyra;  the  bifurcate  nebula  in  Cygnus;  the  Dumb-bell  nebula 
in  Vulpecula;  the  Fan,  Horse-shoe,  Trifid  and  Winged  nebulae  in 
Sagittarius;  the  great  nebula  around  57  Argus  Navis,  and  the  Crab 
nebula  in  Taurus. 

Aside  from  the  discovery  of  some  of  the  largest  nebulae  by  differ- 
ent observers,  and  their  subsequent  arrangement  in  catalogues  by 
Lacaille  and  Messier,  very  little  had  been  done  towards  the  study 
of  these  objects  before  1779,  when  Sir  W.  Herschel  began  to  observe 
them  with  the  earnestness  of  purpose  which  was  one  of  the  distinct- 
ive points  of  the  character  of  this  great  man.  He  successively  pub- 
lished three  catalogues  in  1786,  1789,  and  1802,  in  which  the  position 
of  2,500  nebulous  objects  was  given.  This  number  was  more  than 
doubled  before  1864,  when  Sir  John  Herschel  published  his  catalogue 


146  THE    TROUVELOT 

of  5,079  nebulae  and  star-clusters.  To  this  long  list  must  be  added 
several  hundred  similar  objects,  since  discovered  by  D'Arrest, 
Stephan,  Gould  and  others.  But,  as  has  been  shown  above,  among* 
the  so-called  nebulae  are  many  star-clusters  which  do  not  properly 
belong  to  the  same  class  of  objects,  it  being  sometimes  impossible 
in  the  present  state  of  our  knowledge  to  know  whether  a  nebulous 
object  belongs  to  one  class  or  to  the  other. 

The  nebulae  exhibit  a  great  variety  of  forms  and  appearances, 
and,  in  accordance  with  their  most  typical  characters,  they  are 
usually  divided  into  several  classes,  which  are :  the  Nebulous  stars, 
the  Circular,  or  Planetary,  the  Elliptical,  the  Annular,  the  Spiral  and 
Irregular  nebulae. 

The  so-called  nebulous  stars  consist  of  a  faint  nebulosity,  usually 
circular,  surrounding  a  bright  and  sharp  star,  which  generally  occu- 
pies its  centre.  The  nebulosity  surrounding  these  stars  varies  in 
brightness  as  well  as  in  extent,  and  while,  in  general,  its  light  gradu- 
ally fades  away,  it  sometimes  terminates  quite  suddenly.  Such 
nebulosities  are  usually  brighter  and  more  condensed  towards  the 
central  star.  The  stars  thus  surrounded  do  not  seem,  however,  to 
be  distinguished  from  others  by  any  additional  peculiarity.  Some 
nebulae  of  this  kind  are  round,  with  one  star  in  the  centre  ;  others 
are  oval  and  have  two  stars,  one  at  each  of  their  foci.  The  nebulous 
star,  t  Orionis,  represented  at  the  upper  part  of  Plate  XV.,  above  the 
great  nebula,  has  a  bright  star  at  its  centre  and  two  smaller  ones  on 
the  side.  The  association  of  double  stars  with  nebulae  is  very  re- 
markable, and  may  in  some  cases  indicate  a  mutual  relation  between 
them. 

The  so-called  planetary  nebulae  derive  their  name  from  their 
likeness  to  the  planets,  which  they  resemble  in  a  more  or  less  equa- 
ble distribution  of  light  and  in  their  round  or  slightly  oval  form. 
While  some  of  them  have  edges  comparatively  sharp  and  well  de- 
fined, the  outlines  of  others  are  more  hazy  and  diffused.  These 
nebulae,  which  are  frequently  of  a  bluish  tint,  are  comparatively  rare 
objects,  and  most  of  those  known  belong  to  the  southern  hemisphere. 
When  seen  through  large  telescopes,  however,  they  present  a  differ- 
ent aspect,  and  their  apparent  uniformity  changes.  The  largest  of 
these  objects,  No.  2,343  of  the  General  Catalogue,  is  situated  in  the 
Great  Bear,  close  to  the  star  /?.  Its  apparent  diameter  is,  accord- 
ing to  Sir  J.  Herschel,  2',  40",  and  "  its  light  is  equable,  except  at  the 
edge,  where  it  is  a  little  hazy."  In  a  study  which  I  made  of  this  ob- 
ject in  1876,  with  a  refractor  6%  inches  in  aperture,  I  found  it  decid- 
edly brighter  on  the  preceding  side,  where  the  brightest  part  is 


ASTRONOMICAL    DRA  WINGS.  147 

crescent  shaped.  In  Lord  Rosse's  telescope  its  disk  is  transformed 
into  a  luminous  ring  with  a  fringed  border,  and  two  small  star-like 
condensations  are  found  within.  Another  planetary  nebula,  near  x 
Andromedae,  has  also  shown  an  annular  structure  in  Rosse's  telescope. 

The  elliptical  nebula,  as  their  name  implies,  are  elongated,  ellip- 
tical objects  ;  but  while  some  of  them  are  only  slightly  elongated 
ovals,  others  form  ellipses  whose  eccentricity  is  so  great  that  they 
appear  almost  linear.  In  all  these  objects  the  light  is  more  or  less 
condensed  towards  the  centre  ;  but  while  in  some  of  them  the  con- 
densation is  gradual  and  slight,  in  others  it  is  so  great  and  sudden 
that  the  centre  of  the  nebula  appears  as  a  large  diffused  star,  some- 
what resembling  the  nucleus  of  a  comet.  From  the  general  appear- 
ance of  these  objects,  it  is  not  unlikely  that  some  of  them  are  either 
flattish,  nebulous  disks,  like  the  planetary  nebulae,  or  nebulous  rings, 
seen  more  or  less  sidewise.  The  condensation  of  light  at  their  cen- 
tres does  not  appear  to  be  stellar,  but  nebulous  like  the  rest,  and  it 
is  a  remarkable  fact  that  very  few,  if  any,  of  these  objects  are  resolv- 
able into  stars. 

Several  elliptical  nebulae  are  remarkable  for  having  a  star  at  or 
near  each  of  their  foci,  or  at  each  of  their  extremities.  Such  are  the 
elliptical  nebulae  in  Draco,  Centaurus,  and  Sagittarius,  Nos.  4,419, 
3,706  and  4,395  of  the  General  Catalogue,  the  last  of  which  is  in  the 
vicinity  of  the  triple  star  /Jt  Sagitarii.  Each  of  these  nebulae  has  a 
star  at  each  of  its  foci,  while  No.  I,  in  Cetus,  has  a  star  at  each  of 
its  extremities. 

Among  the  most  remarkable  elliptic  nebulae  may  be  mentioned 
Nos.  1,861  and  2,373  of  Sir  J.  Herschel's  catalogue,  both  situated  in 
the  constellation  Leo.  The  first  is  one  of  Lord  Rosse's  spiral  nebu- 
lae, and  the  last,  which  is  a  very  elongated  object,  is  formed  of  con- 
centric oval  rings,  which  are  especially  visible  towards  its  central 
part.  The  constellation  Draco  is  particularly  remarkable  for  the 
number  of  elliptical  nebulae  found  within  its  boundaries.  Among 
them  are  Nos.  3,939,  4,058,  4,064,  4,087,  4,415,  etc.,  which  are  quite 
remarkable  objects  of  their  class.  No.  4,058,  of  which  I  have  made 
a  study,  is  bright,  and  has  a  decided  lenticular  form  with  a  condensa- 
tion in  the  centre.  Its  following  edge  is  better  defined  than  the 
preceding.  In  Lord  Rosse's  telescope  this  object  exhibits  a  narrow, 
dark,  longitudinal,  gap  in  its  interior. 

By  far  the  largest  and  the  finest  object  of  this  class  is  the  great 
nebula  in  Andromeda.  Although  this  object  belongs  rather  to  the 
class  of  irregular  nebulae,  yet  it  is  generally  considered  as  an  elliptic 
nebula,  since  its  complicated  structure,  being  less  prominent,  was  not 


148  THE    TROUVELOT 

recognized  until  1848,  when  it  was  perceived  by  George  P.  Bond,  Di- 
rector of  the  Harvard  College  Observatory.  This,  the  first  nebula  dis- 
covered, was  found  in  1612  by  Simon  Marius.  It  is  situated  in  the 
constellation  Andromeda,  in  the  vicinity  of  the  star  v,  and  almost  in 
a  line  with  the  stars  //  and  /9  of  the  same  constellation.  It  is  visible 
to  the  naked  eye,  and  appears  as  a  faint  comet-like  object.  It  is 
represented  at  the  upper  left  hand  corner  of  Plate  XIII. ,  on  the 
border  of  the  Milky-way,  as  it  appears  to  the  naked  eye. 

The  nebula  in  Andromeda  is  one  of  the  brightest  in  the  heavens, 
and  is  closely  attended  by  two  smaller  nebulae.  Perhaps  it  would  be 
rather  more  correct  to  say  that  it  has  three  centres  of  condensation, 
as  the  two  small  nebulae  referred  to  are  entirely  involved  in  the 
same  faint  and  extensive  nebulosity.  Its  general  form  is  that  of  an 
irregular  oval,  upwards  of  one  degree  in  breadth  and  two  and  a 
half  degrees  in  length.  Its  brightest  and  most  prominent  part, 
which  alone  was  seen  by  the  earlier  observers,  consists  in  a  very 
elongated  lenticular  mass,  which  gradually  condenses  towards  its 
centre  into  a  blazing,  star-like  nucleus,  surrounded  by  a  brilliant 
nebulous  mass.  At  a  little  distance  to  the  south  of  this  cen- 
tral condensation  is  found  one  of  the  lesser  centres  of  condensa- 
tion noted  above,  which  is  globular  in  appearance,  with  a  bright, 
star-like  nucleus  like  the  former.  The  other  centre  of  condensa- 
tion is  found  to  the  north-west  of  the  centre  of  the  principal 
mass,  and  is  quite  elongated,  with  a  centre  of  condensation  to- 
wards its  southern  extremity,  but  it  is  not  so  bright  as  the  others. 
Close  to  the  western  edge  of  the  bright  lenticular  mass  first  de- 
scribed, and  making  a  very  slight  angle  with  its  longer  axis,  are 
found  two  narrow  and  nearly  rectilinear  dark  rifts,  running  almost 
parallel  to  each  other,  and  both  terminating  in  a  slender  point  in 
the  south.  These  dark  rifts,  which  are  almost  totally  devoid  of 
nebulous  matter,  are  quite  rare  in  nebulae,  and  afford  a  good  oppor- 
tunity to  watch  the  changes  which  this  part  of  the  nebulae  may 
undergo. 

This  nebula  has  never  been  positively  resolved  into  stars,  al- 
though Prof.  Geo.  Bond  and  others  have  strongly  suspected  its 
resolvability.  In  a  study  which  I  have  made  of  it,  with  the  same  in- 
strument employed  by  Bond,  and  also  with  the  great  Washington 
telescope,  I  detected  a  decided  mottled  appearance  in  several 
places,  which  might  be  attributable  to  a  beginning  of  resolvability  ; 
but  I  do  not  consider  this  a  conclusive  indication  that  the  nebula  is 
resolvable.  The  continuous  spectrum  given  by  this  nebula,  showing 
that  it  is  not  in  the  gaseous  state  which  its  appearance  seems  to 


ASTRONOMICAL    DRAWINGS.  149 

indicate,  warrants  the  conclusion,  however,  that  it  will  ultimately 
be  found  to  be  resolvable.  This  object,  being  situated  on  the  edge 
of  the  Galaxy  and  involved  in  its  diffused  light,  has  a  great  number 
of  small  stars  belonging  to  this  belt  projected  upon  it.  During  my 
observations  I  have  mapped  out  1,323  of  these  stars,  none  of  which 
seems  to  be  in  physical  connection  with  the  nebula. 

Among  the  circular  and  elliptical  nebulae  a  few  exhibit  a  very 
remarkable  structure,  being  apparently  perforated,  and  forming 
either  round,  slightly  oval,  or  elongated  rings  of  great  beauty. 
These  Annular  nebulae  are  among  the  rarest  objects  in  the  heavens. 
In  Scorpio,  two  such  nebulae  are  found  involved  in  the  Milky-way, 
and  also  one  in  Cygnus.  One  of  those  in  Scorpio  has  two  stars  in- 
volved within  its  ring,  at  the  extremities  of  its  smallest  interior 
diameter.  A  very  elongated  nebula  in  the  vicinity  of  the  fine  triple 
star  f  Andromedae  is  also  annular,  and  has  two  stars  symmetrically 
placed  at  the  extremities  of  its  greatest  interior  axis.  Another 
elongated  annular  nebula  is  also  found  north  of  rt  Pegasi. 

The  grandest  and  most  remarkable  of  the  annular  nebulae  is 
found  in  the  constellation  Lyra,  about  midway  between  the  two 
stars  $  and  f.  It  is  slightly  elliptical  in  form,  and  according  to 
Prof.  E.  S.  Holden,  its  major  axis  is  77". 3  and  the  minor  58".  From 
a  study  and  several  drawings  which  I  have  made  of  this  object,  with 
instruments  of  various  apertures,  I  have  found  it  decidedly  brighter 
towards  its  outer  border,  at  the  extremities  of  its  minor  axis,  than 
at  the  ends  of  the  major  axis.  On  very  favorable  occasions,  some 
of  its  brightest  parts  have  appeared  decidedly,  but  very  faintly 
mottled,  and  I  have  recognized  three  small  centres  of  condensation. 
Its  interior,  in  which  Professor  Holden  has  detected  a  very  faint  star, 
is  quite  strongly  nebulous.  In  Lord  Rosse's  telescope,  this  nebula 
is  completely  surrounded  by  wisps  and  appendages  of  all  sorts  of 
forms,  which  I  have  failed  to  trace,  however,  both  with  the  refractor 
of  the  Harvard  College  Observatory  and  with  that  of  the  Naval 
Observatory  at  Washington ;  Rosse,  Secchi  and  Chacornac,  have 
seen  this  nebula  glittering  as  if  it  were  a  "  heap  of  star  dust,"  al- 
though its  spectrum  indicates  that  it  is  gaseous. 

The  nebula  No.  1,541,  in  Camelopardus,  of  which  I  have  also  made 
a  study  and  a  drawing,  is  closely  allied  to  the  class  of  annular  neb- 
ulae. This  object,  which  is  quite  bright,  has  a  remarkable  appear- 
ance. It  consists  principally  of  somewhat  more  than  half  of  an 
oval  ring,  surrounding  a  bright,  nebulous  mass  which  condenses 
around  a  star;  this  mass  being  separated  from  the  imperfect  ring  by 
a  dark  interval.  Upon  the  bright  portion  of  the  ring,  and  on  oppo- 


150  THE    TROL  VELOT 

site  points,  are  found  two  bright  stars,  between  which  lies  the  star 
occupying  the  central  mass.  The  central  mass  extends  at  some  dis- 
tance outside  of  the  ring  on  its  open  side.  Several  stars  are  involved 
in  this  object. 

The  Spiral  nebulae  are  very  curious  and  complicated  objects,  but 
they  are  visible  only  in  the  largest  telescopes.  Prominent  above  all 
is  the  double  spiral  nebula  No.  3,572,  in  Canes  Venatici,  which  is 
not  far  from  y  Ursa  Majoris.  In  Lord  Rosse's  telescope,  this  object 
presents  a  wonderful  spiral  disposition,  looking  somewhat  like  one 
of  the  fire-works  called  pin-wheels,  and  forming  long,  curved  wisps, 
diverging  from  two  bright  centres.  The  spectrum  of  this  object, 
however,  is  not  that  of  a  gas.  In  the  constellation  Virgo,  Rosse 
has  detected  another  such  nebula.  In  Cepheus,  Triangulum,  and 
Ursa  Major,  are  found  other  spiral  nebulae  of  smaller  size.  Lord 
Rosse  has  recognized  40  spiral  nebulae  and  suspected  a  similar 
structure  in  30  others. 

The  class  of  the  Irregular  nebulae,  which  will  be  now  considered, 
differs  greatly  in  character  from  the  others,  and  includes  the  largest, 
the  brightest  and  the  most  extraordinary  nebulae  in  the  heavens. 
The  nebulae  of  this  class  differ  from  those  belonging  to  the  other 
classes  by  a  want  of  symmetry  in  their  form  and  in  the  distribution 
of  their  light,  as  well  as  by  their  capricious  shapes,  and  their  very 
complicated  structure.  Another  and  perhaps  the  principal  difference 
between  them  and  the  objects  above  described,  consists  in  the  re- 
markable fact  already  stated,  that  they  are  not,  except  in  rare  cases, 
to  be  found  in  the  regions  where  the  other  nebulae  abound.  On  the 
contrary,  they  are  found  in  or  very  near  the  Milky-way,  precisely 
where  the  other  nebulae  are  the  most  rare.  This  fact,  recognized  by 
Sir  J.  Herschel,  led  him  to  consider  them  as  "  outlying,  very  distant, 
and  as  it  were  detached  fragments  of  the  great  stratum  of  the  Gal- 
axy." It  seems  very  probable  that  the  reason  why  these  objects  dif- 
fer so  greatly  from  the  other  nebulae  in  size,  brightness  and  complica- 
tion of  structure,  is  simply  because  they  are  much  nearer  to  us  than 
are  most  of  the  others.  They  are  perhaps  nebulous  members  of  our 
Galaxy.  The  same  remark  which  has  been  made  of  star-clusters  may 
be  applied  to  nebulae.  The  nearer  they  are  to  us,  the  larger,  the 
brighter  and  the  more  complicated  they  will  appear,  while  the  far- 
ther they  are  removed,  the  more  simple  and  regular  and  round 
they  will  appear,  only  their  brightest  and  deepest  parts  being  then 
visible. 

The  Crab  Nebula  of  Lord  Rosse,  near  ;  Tauri,  No.  1,157,  is  one 
of  the  interesting  objects  of  this  class.  It  has  curious  appendages 


A STRONOMICA L    DRA  WINGS.  1 51 

streaming  off  from  an  oval,  luminous  'mass,  which  give  it  a  distant 
resemblance  to  the  animal  from  which  it  derives  its  name.  The 
Bifid  nebula  in  Cygnus,  Nos.  4,400  and  4,616,  is  another  object  of 
this  class.  It  consists  of  a  long,  narrow,  crooked  streak,  forking  out 
at  several  places,  and  passing  through  x  Cygni.  Observers,  having 
failed  to  recognize  the  connection  existing  between  its  different 
centres  of  brightness,  have  made  distinct  nebulae  of  this  extended 
object. 

The  Dumb-bell  nebula  in  Vulpecula,  No.  4,532,  is  a  bright  and 
curious  object,  with  a  general  resemblance  to  the  instrument  from 
which  it  derives  its  name.  Lord  Rosse's  telescope  has  shown  many 
stars  in  it,  projected  on  a  nebulous  background,  and  Prof.  Bond 
seems  to  have  thought  that  it  showed  traces  of  resolvability,  al- 
though in  the  study  which  I  made  of  this  nebula  with  the  same  in- 
strument used  by  the  latter  observer,  I  failed  to  perceive  any  such 
traces.  Dr.  Huggins  finds  its  spectrum  gaseous. 

The  star-cluster,  No.  4,400,  in  Scutum  Sobieskii,  which  is  de- 
scribed by  Sir  J.  Herschel  as  a  loose  cluster  of  at  least  100  stars,  I 
have  found  to  be  involved  in  an  extensive,  although  not  very  bright, 
nebula,  which  would  seem  to  have  escaped  his  scrutiny.  In  a  study 
and  drawing  of  this  nebula  made  in  1876,  its  general  form  is  that 
of  an  open  fan,  with  the  exception  that  the  handle  is  wanting,  with 
deeply  indented  branches  on  the  preceding  side,  where  the  brightest 
stars  of  the  cluster  are  grouped.  From  its  peculiar  form,  this  object 
might  appropriately  be  called  the  Fan  nebula. 

The  Omega  or  Horse-shoe  nebula,  in  Sagittarius,  No.  4,403,  of 
which  I  have  made  a  study  and  two  drawings,  one  with  a  refractor  6^ 
inches  in  aperture,  and  the  other  conjointly  with  Prof.  Holden,  with 
the  great  telescope  of  the  Naval  Observatory,  is  a  bright  and  very 
complicated  object.  Its  general  appearance  in  small  instruments,, 
with  low  power,  is  that  of  a  long,  narrow  pisciform  mass  of  light, 
from  which  proceeds  on  the  preceding  side,  the  great  double 
loop  from  which  it  derives  its  name.  But  in  the  great  Washing- 
ton refractor  its  structure  becomes  very  complicated,  forming  vari- 
ous bright  nebulous  masses  and  wisps  of  great  extension.  Prof. 
Holden,  who  has  made  a  careful,  comparative  study  of  the  pub- 
lished drawings  of  this  object,  thinks  there  are  reasons  to  believe 
that  its  western  branch  has  moved  relatively  to  the  stars  found 
within  its  loop.  The  spectrum  of  this  nebula  is  gaseous. 

The  Trifid  nebula,  No.  4,355,  in  the  same  constellation,  is  also  a 
very  remarkable  object,  although  it  is  not  so  bright  as  the  last. 
This  nebula,  which  I  have  studied  with  the  refractor  of  the  Cam- 


152  THE    T^OUVELOT 

bridge  Observatory,  consists  of  four  principal  masses  of  light,  separ- 
ated by  a  wide  and  irregular  gap  branching  out  in  several  places. 
These  masses,  which  are  brighter  along  the  dark  gap,  gradually 
fade  away  externally.  A  group  of  stars,  two  of  which  are  quite 
bright,  is  found  near  the  centre  of  the  nebula,  on  the  inner  edge  of 
the  following  mass,  and  close  to  the  principal  branch  of  the  dark- 
channel.  A  little  to  the  north,  and  apparently  forming  a  part  of 
this  nebula,  is  a  globular-looking  nebula,  having  a  pale  yellow  star 
at  its  centre.  Prof.  Holden's  studies  on  this  nebula  show  that  the 
triple  star,  which  was  centrally  situated  in  the  dark  gap  from  1784 
to  1833,  was  found  involved  in  the  border  of  the  nebulous  mass  fol- 
lowing it,  from  1839  to  l877  5  the  change,  he  thinks,  is  attributable 
either  to  the  proper  motion  of  the  group  of  stars  or  to  that  of  the 
nebula  itself. 

In  the  same  vicinity  is  found  the  splendid  and  very  extensive 
nebula  No.  4,361,  in  which  is  involved  a  loose,  but  very  brilliant 
star-cluster.  This  nebula  and  cluster,  which  I  have  studied  and 
drawn  with  a  6^3  inch  telescope,  is  very  complicated  in  structure, 
and  divided  by  a  dark  irregular  gap  into  three  principal  masses  of 
light,  condensing  at  one  point  around  a  star,  and  at  others  forming 
long,  bright,  gently-curved  branches,  which  give  to  this  object  a 
strong  resemblance  to  the  wings  of  a  bird  when  extended  upwards 
in  the  action  of  flying.  From  this  peculiarity  this  object  might  ap- 
propriately be  called  the  Winged  nebula.  Its  spectrum  is  that  of  a 
gas. 

The  variable  star  rt  Argus  is  completely  surrounded  by  the  great 
nebula  of  the  same  name,  No.  2,197,  ^rst  delineated  by  Sir  J. 
Herschel,  during  his  residence  at  the  Cape  of  Good  Hope,  in  1838. 
This  object,  which  covers  more  than  f  of  a  square  degree,  is  divided 
into  three  unequal  masses,  separated  by  dark  oval  spots,  compara- 
tively free  from  nebulosity,  and  is  suspected  to  have  undergone 
changes  since  Herschel's  time. 

In  the  same  field  with  the  double  star,  £  Orionis,  the  most  east- 
erly of  the  three  bright  stars  in  the  belt  of  Orion,  is  found  another 
irregular  nebula  of  the  Trifid  type.  From  the  drawings  which  I 
have  made  of  this  object,  it  appears  to  be  composed  of  three  princi- 
pal unequal  masses,  separated  by  a  wide,  irregular,  dark  channel, 
two  of  the  masses  being  quite  complicated  in  structure,  and  forming 
curved,  nebulous  streams  of  considerable  length  and  breadth.  This 
nebula,  like  the  next  to  be  described,  seems  to  be  connected  with 
the  Galaxy  by  the  great  galactic  loop  described  in  another  section. 

By   far  the  most  conspicuous  irregular  nebula  visible  from  our 


ASTRONOMICAL    DRA  WINGS.  153 

northern  States,  is  the  great  nebula  in  Orion,  No.  1,179,  repre- 
sented on  Plate  XV.  This  object,  visible  to  the  naked  eye,  is  the 
brightest  and  the  most  wonderful  nebula  in  the  heavens.  It  is  situ- 
ated a  little  to  the  south  of  the  three  bright  stars  in  the  belt  of 
Orion,  and  may  be  readily  detected  surrounding  the  star  6,  situated 
between  and  in  a  line  with  two  faint  stars,  the  three  being  in  a 
straight  line  which  points  directly  towards  e,  the  middle  star  of  the 
three  in  Orion's  belt.  The  area  occupied  by  this  nebula  is  about 
equal  to  that  occupied  by  the  Moon. 

In  its  brightest  parts  the  nebula  in  Orion  appears  as  a  luminous 
cloud  of  a  pentagonal  form,  from  which  issue  many  luminous  ap- 
pendages of  various  shapes  and  lengths.  This  principal  mass  is 
divided  into  secondary  masses,  separated  by  darkish,  irregular  inter- 
vals: These  secondary  masses  in  their  turn  appear  mottled  and 
fleecy.  Towards  the  lower  part  of  the  pentagonal  mass  is  found  a 
roundish  dark  space,  comparatively  devoid  of  nebulosity,  in  which 
are  involved  four  bright  stars  forming  a  trapezium,  and  several 
fainter  ones.  The  four  bright  stars  of  the  trapezium  constitute  the 
quadruple  star  0  Orionis,  from  which  the  nebula  has  received  its 
name.  The  cloud-like  pentagonal  form  is  brightest  on  the  north- 
west of  the  trapezium,  and  is  surrounded  on  three  sides  by  long, 
soft,  curved  wisps,  fading  insensibly  into  the  outer  nebulous  mass  in 
which  they  are  involved.  On  the  east  a  broad,  wavy  wing  spreads 
out,  and  sends  an  important  branch  southward.  South-east  of  the 
trapezium  are  found  several  curious  dark  spaces,  comparatively  de- 
void of  nebulosity,  especially  those  on  the  east,  which  give  to  this 
nebula  a  singular  character.  Close  to  the  north-eastern  part  of 
the  nebula,  or  rather  in  contact  with  it,  is  found  a  small,  curiously- 
shaped  nebula,  condensing  around  a  bright  star  into  a  blazing  nu- 
cleus. From  this  centre  it  continues  northward  in  a  narrow  diffused 
stream,  which  spreads  out  in  passing  over  the  stars  cl  and  c2;  and 
after  having  sent  short  branches  northward,  it  curves  back  to  the 
south  and  joins  the  main  nebula  on  the  west  of  its  starting  point, 
having  thus  formed  a  great  loop  which  is  not  shown  on  the  Plate. 
The  nebula  also  forms  a  loop  towards  the  south,  which  is  partly 
shown  on  Plate  XV.,  a  small  branch  of  which,  passing  through  t 
Orionis,  the  nebulous  star  shown  at  the  top  of  the  Plate,  and  ex- 
tending southward,  is  not  here  represented. 

On  ordinary  nights  the  nebula  in  Orion  is  a  splendid  object,  and 
inspires  the  observer  with  amazement;  but  this  is  as  nothing  com- 
pared with  the  grand  and  magnificent  sight  which  it  presents  dur- 
ing the  very  rare  moments  when  our  atmosphere  is  perfectly  clear 


154  THE     TROUVELOT 

and  steady.  I  have  seen  this  nebula  but  once  under  these  favorable 
circumstances,  and  I  was  surprised  by  the  grandeur  of  the  scene. 
Then  could  be  detected  features  to  be  seen  at  no  other  time,  and  its 
fleecy,  floculent,  cloud-like  masses  glittered  with  such  intensity  that 
it  seemed  as  if  thousands  of  stars  were  going  to  blaze  out  the  next 
moment.  Although  I  observed  the  nebula  under  such  favorable 
conditions,  and  with  the  fifteen-inch  refractor  of  the  Cambridge  Ob- 
servatory, yet  I  was  disappointed  in  my  expectations,  and  distin- 
guished no  new  stars  or  points  of  light,  and  nothing  more  than  a  very 
bright  mass,  finely  divided  into  minute  blazing  cloudlets.  Although 
I  failed  to  resolve  this  nebula  into  stars,  yet  Lord  Rosse,  Bond 
and  Secchi  thought  they  had  caught  glimpses  of  star  dust.  Its 
spectrum,  however,  proves  to  be  mainly  that  of  incandescent  gases, 
probably  hydrogen  and  nitrogen.  In  the  curved  wisps  found  in  this 
nebula,  Lord  Rosse  and  others  saw  indications  of  a  spiral  structure. 

Several  bright  stars  are  found  scattered  over  this  nebula,  and  be- 
sides those  forming  the  trapezium,  there  are  three  in  a  row,  a  little 
to  the  south-east  of  that  group,  which  are  quite  bright  and  remark- 
able. Among  the  stars  involved  in  this  nebula,  few  show  signs  of 
having  a  physical  connection  with  it,  although  it  seems  probable 
that  the  group  of  the  trapezium  is  so  connected.  Some  of  these 
stars  are  variable.  The  small  stars  represented  on  this  Plate,  as  on 
others  of  the  series,  are  somewhat  exaggerated  in  size,  as  was  un- 
avoidable with  any  process  of  reproduction  which  could  be  adopted. 

In  1811,  W.  Herschel  was  led  to  suspect  that  some  changes  had 
occurred  in  this  nebula,  but  changes  in  such  complicated  and  delicate 
objects  are  not  easily  ascertained,  since,  for  the  most  part,  we  have 
for  comparison  with  our  later  observations  only  coarse  drawings 
made  by  hands  unskilled  in  delineation. 

Although  comparatively  rare,  double  and  multiple  nebulae  may 
be  found  in  the  sky.  When  this  occurs,  their  constituents  most 
commonly  belong  to  the  class  of  spherical  nebulae.  Sometimes  the 
components  are  separated  and  distinct,  at  other  times  one  of  them  is 
projected  upon  the  other,  either  really  or  by  the  effect  of  perspect- 
ive. Sometimes  one  is  round  and  the  other  elongated.  It  is  proba- 
ble that  while  some  of  these  nebulae  are  physically  associated  and 
form  a  system,  others  appear  to  be  so  only  because  they  happen  to 
be  almost  in  a  line  with  the  observer.  A  double  nebula  in  Draco, 
Nos.  4,127  and  4,128,  which  I  have  drawn,  is  a  fair  type  of  those 
which  are  separated.  The  first  is  a  globular  nebula,  and  the  last  an 
oval  one,  with  a  star  at  its  centre.  The  double  nebula,  Nos.  858  and 
859,  in  Taurus,  which  I  have  also  studied,  is  a  type  of  the  cases  in 


ASTRONOMICAL     DRAWINGS.  155 

which  one   nebula  is  partly  projected  on  another.     In  this  instance 
both  the  nebulae  are  globular. 

The  nebulae  in  general  show  very  little  color  in  their  light, 
which  is  ordinarily  whitish  and  pale.  Some,  however,  present  a 
decided  bluish  or  greenish  tint.  The  great  nebula  in  Orion  has  a 
greenish  cast,  and  we  have  seen  that  some  planetary  nebulae  are 
bluish. 

It  has  been  a  question  whether  nebulae  are  changing.  It  has 
already  been  stated  that  Prof.  Holden  believes  there  is  ground  to 
suspect  that  the  Trifid  and  Horse-shoe  nebulae  have  undergone  some 
changes.  A  nebula  near  £  Tauri  has  been  lost  and  found  again 
several  times.  Two  other  nebulae  in  the  same  constellation  have 
presented  curious  variations.  One,  near  a  star  of  the  tenth  magni- 
tude, exhibited  variations  of  brightness  like  those  of  the  star  itself, 
and  for  a  time  disappeared.  The  other,  near  f  Tauri,  increased  in 
brightness  for  three  months,  after  which  it  disappeared.  In  1859 
Tempel  discovered  a  nebula  in  the  Pleiades,  which  has  shown  some 
fluctuations.  In  1875  I  made  a  long  study  of  this  object,  and  drew 
it  carefully  a  dozen  times,  but  I  was  not  able  to  see  any  changes  in 
it  within  the  two  or  three  months  during  which  my  observations  were 
continued.  But  on  Nov.  24,  1876,  it  was  found  of  a  different  color, 
being  purplish  and  very  faint.  On  Dec.  23,  1880,  it  was  found  just  as 
bright  and  visible  as  when  I  drew  it  in  1875,  an^  on  Oct.  20,  1881, 
it  appeared  faint  and  purplish  again,  as  in  1876.  On  this  last  night, 
and  on  those  which  followed  it,  it  was  impossible  for  me  to  trace 
the  nebulosity  as  far  as  in  1875.  I  consider  this  as  due  to  a  vari- 
ation in  the  light  of  this  object,  which  in  1875  was  bright  enough 
to  be  well  seen  while  the  Moon  after  her  First  Quarter  was  within 
ten  or  fifteen  degrees  from  the  Pleiades. 

From  the  observations  of  M.  Laugier,  it  appears  that  some 
nebulae  have  a  proper  motion,  comparable  to  that  of  stars.  From 
the  displacement  of  the  lines  of  their  spectra  by  their  motion  in  the 
line  of  sight,  Dr.  Huggins  found  that  no  nebula  observed  by  him  has 
a  proper  motion  surpassing  25  miles  per  second.  The  Ring  nebula 
in  Lyra  appears,  to  move  from  us  at  the  rate  of  3  miles  per  second, 
and  that  in  Orion  recedes  about  17  miles  per  second. 

The  important  question  arises,  are  all  the  irresolvable  nebulas 
in  the  heavens  to  be  considered  as  so  many  star-clusters,  differing 
only  from  them  by  the'  minuteness  of  their  components,  or  their 
immense  distance  from  us;  or  are  they  cosmical  clouds,  composed 
of  luminous  vapors,  similar  to  the  matter  composing  the  heads 
and  tails  of  comets  ?  Originally,  W.  Herschel,  with  many  as- 


156  THE     TROUVELOT 

tronomers,  thought  that  all  these  objects  were  stellar  aggregations, 
too  distant  to  be  resolved  into  stars;  but  he  subsequently  modified 
his  opinion,  and  accepted  the  idea  that  some  of  them  are  of  a  gaseous 
nature. 

No  direct  proof  that  the  nebulae  are  gaseous  could  be  obtained, 
however,  before  the  spectroscope  was  known.  The  attempt  to 
analyze  the  light  of  the  nebulas  with  this  instrument  was  made  in 
1864,  by  Dr.  Huggins,  who  directed  his  spectroscope  to  the  planet- 
ary nebula,  No.  4,373,  in  Draco.  Its  spectrum  was  found  to  consist 
of  three  bright,  distinct  lines,  the  brightest  of  which  corresponded 
with  the  strongest  nitrogen  line,  and  the  feeblest  with  the  hydro- 
gen C  line.  Besides  these  lines,  it  gave  also  a  very  faint,  continu- 
ous spectrum,  apparently  due  to  a  central  point  of  condensation. 
By  this  observation,  the  gaseous  nature  of  a  nebula  was  for  the  first 
time  demonstrated.  Dr.  Huggins  thus  analyzed  70  nebulae,  of 
which  one-third  gave  a  gaseous  spectrum,  consisting  6f  several 
bright  lines,  the  brightest  of  which  invariably  corresponded  with 
the  lines  of  nitrogen.  The  others  gave  a  continuous  spectrum, 
with  the  red  end  usually  deficient.  These  results  indicate  that  if 
some  of  the  so-called  nebulae  are  due  to  an  aggregation  of  stars, 
either  too  minute  or  too  remote  in  space  to  be  individually  resolved, 
others  are  in  a  gaseous  state.  Yet  the  faint,  continuous  spectrum, 
given  by  some  nebulae,  in  addition  to  their  gaseous  spectra,  seems 
to  show  that  these  nebulae  have  some  stars  or  matter  in  a  different 
state,  either  involved  in  them  or  projected  on  their  surface. 

The  idea  of  diffused  matter  distributed  here  and  there  in  space, 
and  gradually  condensing  into  stars,  is  by  no  means  new.  As  early 
as  1572,  Tycho  Brahe  proposed  such  an  hypothesis,  to  explain  the 
sudden  apparition  of  a  new  star  in  Cassiopeia,  which  he  considered 
as  formed  by  the  recent  agglomeration  of  the  "  celestial  matter " 
diffused  in  space.  Kepler  adopted  the  same  idea  to  explain  the 
new  star  which  appeared  in  Ophiuchus,  in  1604.  Halley,  Lacaille, 
Mairan  and  others,  entertained  the  same  opinion.  The  hypothesis 
of  a  self-luminous,  nebulous  matter  diffused  in  space,  and  forming 
here  and  there  immense  masses,  has  been  proposed  from  the  origin 
of  the  telescope,  and  was  adopted  by  Sir  William  Herschel,  who  in 
his  grand  speculations  on  the  universe  considered  the  nebulae  as 
immense  masses  of  phosphorescent  vapors,  gradually  condensing 
around  one  or  several  centres  into  stars  or  clusters  of  stars.  The 
evidence  afforded  by  the  spectroscope  seems  to  be  in  favor  of  such 
an  hypothesis,  and  shows  us  that  gaseous  agglomerations  exist  in 
space. 


ASTRONOMICAL     DRAWINGS.  157 

According  to  our  modern  conception,  the  visible  universe  is  but 
an  infinitely  small  portion  of  the  infinite  universe  perceived  by  our 
mind.  The  great  blazing  centre  around  which  our  little,  non-lumin- 
ous globe  pursues  its  endless  journey,  is  only  an  humble  member  of 
a  cluster  comprising  four  hundred  equally  powerful  suns,  as  they 
are  believed  to  be,  although  they  appear  to  us  as  little  twinkling 
stars.  The  nearest  of  these  stars  is  221,000  times  as  far  from  the 
Sun  as  the  Sun  is  from  the  Earth,  and  yet  this  entire  cluster  is  only 
one  among  the  several  hundred  Star-clusters  composing  the  great 
galactic  nebula  in  which  we  are  involved,  comprising  thirty  or  fifty 
millions  of  such  suns.  Among  the  4,000  irresolvable  nebulae  in  the 
sky,  perhaps  over  one-half  are  supposed  to  be  galaxies,  like  our 
own  galaxy,  composed  of  star-clusters,  and  millions  of  stars.  Be- 
sides these  remote  galaxies,  vast  agglomerations  of  yet  uncon- 
densed,  nebulous  matter  exist  in  space,  and  form  the  nebulae  proper, 
in  which  the  genesis  of  suns  is  slowly  elaborated.  Although  the 
visible  universe  is  limited  by  the  penetrating  power  of  our  instru- 
ments, yet  we  see  in  imagination  the  infinite  universe  stretching 
farther  and  farther;  but  we  know  not  whether  this  invisible  universe 
is  totally  devoid  of  matter,  or  whether  it  also  is  filled  with  millions 
and  millions  of  suns  and  galaxies. 


APPENDIX. 


KEY  TO  THE  PLATES. 

{The  numerals  in  brackets  refer  to  pages  of  the  MANUAL.  The  letters  a,  b,  c,  designate  respectively 
the  upper,  middle  and  lower  portions  of  the  page.] 

PLATE  I.— GKOUP  OF  SUN-SPOTS  AND  VEILED  SPOTS. 

Observed  June  17,  1875,  at  7h.  30m.  A.  M. 

The  background  shows  the  sun's  visible  surface,  or  photosphere  [26],  as  seen  with  a  tele- 
scope of  high  power  at  the  most  favorable  moments,  composed  of  innumerable  light  markings, 
or  granules  [3c],  separated  by  a  network  of  darker  gray.  The  granules,  each  some  hun- 
dreds of  miles  in  width  [4a],  are  thought  to  be  the  flame-like  [5a]  summits  of  the  radial  fila- 
ments or  columns  of  gas  and  vapor  [5c]  which  compose  the  photospheric  shell  [10a,  17a].  The 
two  principal  sun-spots  [8a]  of  the  group  [156]  here  represented  show  the  characteristic  dark 
umbra  in  the  centre  [96],  overhung  by  the  thatch-like  penumbra  [105],  composed  of  whit- 
ish gray  filaments.  The  penumbral  filaments  are  not  supposed  to  differ  in  their  nature  from 
those  constituting  the  ordinary  photosphere,  save  that  they  are  seen  here  elongated  and 
violently  disturbed  by  the  force  of  gaseous  currents  [106].  Both  spots  are  traversed 
partly  or  wholly  by  bright  overlying  faculce  [6a],  or  so-called  luminous  bridges  [lOc],  depressed 
portions  of  which,  in  the  left-hand  spot,  form  the  gray  and  rosy  veils  [9r]  commonly 
attendant  upon  this  class  of  spots  [13c].  In  each  of  these  spots,  also,  the  inner  ends  of  pro- 
jecting penumbral  filaments  have  fallen  so  far  within  the  umbra  [lla]  as  to  appear  much 
darker  than  the  rest.  At  the  right  of  the  upper  portion  of  the  left-hand  spot,  is  a  mass 
of  white  facular  clouds  [146],  honey-combed  by  dark  spaces,  through  which  are  seen  traces 
of  the  undeveloped  third  spot  of  the  triple  group  first  observed  [176].  If  seen  upon  the  sun's 
limb,  this  would  have  presented  the  appearance  of  a  lateral  spot  [136].  Above  the  right-hand 
spot  is  a  small  black  "  dot,"  or  incipient  spot,  without  distinct  penumbra  [96].  The  ir- 
regular dark  rift  below  the  two  large  spots  and  connecting  them  is  a  spot  of  the  crevasse  type 
[14a],  with  very  slight  umbra,  a  still  better  example  of  which  is  seen  in  a  westward  [vi.  c]  pro- 
longation of  the  penumbra  of  the  left-hand  spot.  In  the  upper  left-hand  corner  of  the  Plate 
are  see*n  several  small  faculce  [6a],  appearing  as  irregular  whitish  streaks  amongst  the  gran- 
ules [7c].  In  the  pear-shaped  darkening  of  the  solar  surface  below  and  at  their  left,  is  seen 
A  veue$spot  [IGc],  two  of  which  attended  this  group  [176]. 

Approximate  scale,  2500  mUes—l  inch. 


160  APPENDIX. 

PLATE  II.— SOLAR    PROTUBERANCES. 
Observed  May  5,  1873,  at  9h.  40m.  A.  M. 

A  view  of  an  upheaval  of  the  chromosphere  [18a],  or  third  outlying  envelope  of  tin-  sun 
[3a],  as  observed  with  the  tele-spectroscope,  or  telescope  with  spectroscope  attached. 

The  method  of  the  observation  requires  a  word  of  explanation.  Save  on  the  rare  occasions 
of  a  total  solar  eclipse,  no  direct  telescopic  view  of  the  solar  prominences  or  flames  is  possi- 
ble, owing  to  the  fact  that  the  intense  white  light  from  the  sun's  main  disk  entirely  ob- 
scures [3a]  the  feeble  pink  light  of  the  chromosphere  [26cr].  A  few  years  ago  Messrs. 
Jannsen  and  Lockyer  [18&]  found  that  a  spectroscope  of  high  dispersive  power  so  weakens 
the  spectrum  of  ordinary  sun-light  as  to  show  the  spectrum  of  bright  lines  given  by  the 
chromosphere  [21a],  on  any  clear  day.  The  telescope  is  adjusted  so  that  a  portion  of  the 
sun's  limb,  usually  near  a  group  of  active  sun-spots  [21a],  shall  be  presented  before  the 
opened  slit  of  the  spectroscope.  The  light  of  the  chromosphere  thus  admitted  along  with 
some  diffused  sun-light  from  the  earth's  atmosphere,  produces  a  spectrum  of  intensely  bright 
lines,  widely  separated,  on  the  fainter  background  of  the  strongly  dispersed  spectrum  of 
sun-light.  The  most  prominent  of  these  bright  lines  are  those  known  as  the  C  line  (scarlet), 
Fliue  (blue),  which  with  several  others  are  due  to  the  hydrogen  present  in  the  chromosphere, 
the  Ds  line  (orange)  ascribed  to  a  little  known  substance  called  "helium"  [5a],  and  occasion- 
ally the  sodium  lines  Di,  Do,  (yellow).  By  adjusting  the  slit  upon  the  scarlet  C  line,  the  ap- 
pearances represented  in  Plate  II.  were  observed  as  through  an  atmosphere  of  scarlet  light; 
in  the  D  or  F  lines  identical  appearances  may  be  seen,  but  somewhat  less  clearly  denned,  as 
through  yellow  or  blue  light  respectively.  Hence  the  solar  flames,  as  here  observed  with 
the  spectroscope  in  the  hydrogen  C  line,  are  seen  through  a  portion  only  (the  scarlet  rays)  of 
the  light  coming  from  but  one  substance  (hydrogen)  of  the  companion  incandescent  sub- 
stances present  in  the  chromosphere.  The  color  of  the  collective  chromospheric  light  is  seen 
directly  with  the  telescope  during  an  eclipse  (See  Plate  III.)  to  be  a  delicate  rosy  pink  [26a.J 

Description  of  the  Plate. — The  black  background  represents  the  general  darkness  of  the 
eye-piece  to  the  spectroscope.  *  The  broad  red  stripe  stretching  from  top  to  bottom  of  the 
Plate  is  a  portion  of  the  red  band  of  the  spectrum,  magnified  about  100  times  as  compared 
with  the  actual  spectroscopic  view.  The  upper  and  lower  edges  of  the  cross-section  of  dnsky 
red  correspond  with  the  edges  of  the  slit,  opened  widely  enough  to  admit  a  view  of  the 
chromospheric  crest  and  of  the  whole  height  of  the  protuberances  at  once.  With  a  narrower 
opening  of  the  slit  this  background  would  have  been  nearly  black,  its  reddish  cast  increasing 
with  the  amount  of  opening  and  consequent  admission  of  diffused  sun-light.  Rising  above 
the  lower  edge  of  the  opening  is  seen  a  small  outer  segment  of  the  chromosphere,  which,  as  a 
portion  of  the  sun's  eastern  limb,  should  be  imagined  as  moving  directly  towards  the  be- 
holder. The  seams  and  rifts  by  which  its  surface  is  broken,  as  well  as  the  distorted  forms 
of  the  huge  protuberances  show  the  chromosphere  to  be  in  violent  agitation.  Some  of  the 
most  characteristic  shapes  [J9c]  of  the  eruptive  protuberances  [20a]  are  presented,  as  also 
cloud-like  forms  overtopping  the  rest.  In  the  immediate  foreground  the  bases  of  two  tower- 
ing columns  appear  deeply  depressed  below  the  general  horizon  of  the  segment  observed, 
showing  an  extraordinary  velocity  of  motion  of  the  whole  uplifted  mass  toward  the  observer 
[21c].  The  highest  of  these  protuberances  was  126,000  miles  in  height  at  the  moment  of 
observation  [22a].  The  triple  protuberance  at  the  left  with  two  drooping  wings  [21b]  and  a 
tall  swaying  spire  tipped  with  a  very  bright  flame,  shows  by  its  more  brilliant  color  the  higher 
temperature  (and  possibly  compression)  to  which  its  gases  have  been  subjected  [18c].  The 


KEY  TO  THE  PLATES.  161 

irregular  black  bauds  behind  this  protuberance  indicate  the  presence  there  of  less  con- 
densed and  cooler  clouds  of  the  same  gases  [20c].  The  dimmer  jets  of  flame  rising  from 
the  chromosphere  are  either  vanishing  protuberances  [20c],  or,  as  in  the  case  of  the  smallest 
jet  shown  at  the  extreme  right  of  the  horizon,  are  the  tops  of  protuberances  just  coming 
into  view. 

Approximate  scale,  6000  miles—  1  inch. 

PLATE  III. -TOTAL  ECLIPSE   OF   THE   SUN. 

Observed  July  29,  1878,  at  Oreiton,  Wy.miiny  Territory. 

A  telescopic  view  of  the  sun's  corona  or  extreme  outer  atmosphere  [36]  and  of  the  solar 
flames  or  prominences  [25c]  during  a  total  eclipse  [23c].  At  the  moment  of  observation  the 
dark  disk  of  the  moon,  while  still  hiding  the  sun's  main  body,  had  passed  far  enough  east- 
ward to  allow  the  rosy  pink  chromospheric  prominences  to  be  seen  on  its  western  border 
[2tic].  On  all  sides  of  the  sun's  hidden  disk,  the  corona  [256]  shows  its  pale  greenish  light 
[26a]  extending  in  halo-like  rays  and  streamers,  and  two  very  remarkable  wings  [266]  stretch 
eastward  and  westward  very  nearly  in  the  plane  of  the  ecliptic  and  in  the  direction  of  the 
positions  of  Mercury  and  Venus  respectively  at  the  time  of  observation.  The  full  extent  of 
these  wings  could  not  be  shown  in  the  Plate  without  reducing  its  scale  materially,  since  the 
westerly  wing  extended  no  less  than  twelve  times  the  sun's  diameter  [276],  and  the  easterly 
wing  nearly  as  far,  or  over  ten  million  miles.  A  circlet  of  bright  light  immediately  border- 
ing the  moon's  disk  is  the  so-called  inner  corona,  next  to  which  the  wings  and  streamers  are 
brightest,  thence  shading  off  imperceptibly  into  the  twilight  sky  of  the  eclipse  [256].  Other 
noteworthy  peculiarities  of  the  corona,  as  observed  during  this  ealipse  [265],  are  the  varying 
angles  at  which  the  radiating  streamers  are  seen  to  project,  the  comparatively  dark  intervals 
between  them,  and  the  curved,  wisp-like  projections  seen  upon  the  wings.  An  especially 
noticeable  gap  appears  where  the  most  westerly  of  the  upward  streamers  abruptly  cuts  off 
the  view  of  the  long  wing.  The  largest  and  brightest  of  the  curving  streamers  on  the  west- 
erly wing  coincides  with  the  highest  flame-like  protuberance  [266].  To  some  observers 
of  this  eclipse  the  upward  and  downward  streamers  seemed  pointed  at  their  outer  extremi- 
ties and  less  regular  in  form. 

Approximate  scale,  135,000  miles— I  inch. 

PLATE  IV.— AURORA  BOREALIS. 
As  observed  March  1,  1872,  at  9/t.  25m.  P.  M. 

The  view  presents  the  rare  spectacle  of  an  aurora  spanning  the  sky  from  east  to  west 
in  concentric  arches  [28c].  The  Polar  Star  is  nearly  central  in  the  back-ground,  the  constel- 
lation of  the  Great  Bear  on  the  right  and  Cassiopeia's  chair  on  the  left.  The  large  star  at 
some  distance  above  the  horizon  on  the  right  is  Arcturus.  The  almost  black  inner  segment 
[286]  of  the  aurora  resting  upon  the  horizon,  has  its  summit  in  the  magnetic  meridian  [32a] , 
which  was  in  this  case  a  little  west  of  north,  its  arc  being  indented  by  the  bases  of  the  ascend- 
ing streamers  [28c].  Both  streamers  and  arches  were,  when  observed,  tremulous  with  up- 
ward pulsations  [296]  and  there  was  also  a  wave-like  movement  of  the  streamers  from  west 
to  east  [29a].  The  prevailing  color  of  this  aurora  is  a  pale  whitish  green  [28c]  and  the  com- 
plementary red  appears  especially  at  the  west  end  of  the  auroral  arch.  The  summits  of  the 
streamers  are  from  four  hundred  to  five  hundred  miles  above  the  earth  [31«]  and  the  aurora 
is  therefore  a  phenomenon  of  the  terrestrial  atmosphere  [326]  rather  than  of  astronomical 
observation  proper. 


162  APPENDIX. 

PLATE  V.— THE  ZODIACAL  LIGHT. 

Observed  February  20,  1876. 

An  observation  of  the  cone  of  light  whose  axis  lies  along  the  Zodiac  [36&],  whence  it 
derives  its  name.  It  is  drawn  as  seen  in  the  west  [41a],  with  its  base  in  the  constellation 
Pisces,  and  its  apex  near  the  familiar  group  of  the  Pleiades  in  the  constellation  Taurus.  The 
first  bright  star  above  the  horizon  in  the  base  of  the  cone  is  the  planet  Venus  and  at  some 
distance  above  is  the  reddish  disk  of  Mars,  the  two  being  in  rare  companionship  as  evening 
stars.  Above  the  constellation  Pisces,  two  bright  stars  of  Aries  lie  just  outside  the  cone  at 
the  right.  The  nearest  bright  star  above  these  at  the  right  is  Beta,  the  leading  star  of  the 
constellation  Triangula.  Further  at  the  right  the  three  prominent  stars  nearly  in  a  line  are, 
in  ascending  order,  Delia,  Beta  and  Gamma  of  the  constellation  Andromeda.  Above  these 
at  the  left,  the  brightest  star  of  a  quadrangular  group  of  four  is  the  remarkable  variable  star 
Algol  (Beta)  of  the  constellation  Perseus,  which  changes  from  the  second  to  the  fourth  mag- 
nitude in  a  period  of  less  than  three  days.  At  the  left  and  a  little  above  the  Pleiades  is  the 
ruddy  star  Aldebaran,  one  of  the  Hyades  and  chief  star  in  the  constellation  Taurus.  These 
are  the  principal  stars  visible  in  this  portion  of  the  sky  at  the  time  of  the  observation.  Their 
relative  positions  are  represented  as  seen  in  the  sky  and  not  by  the  common  method  of  star- 
atlases,  which  allows  for  the  change  from  a  spherical  to  a  plane  surface.  Their  magnitude  in 
the  order  of  brightness  is  indicated  only  approximately  [41a]. 

PLATE  VI.— MAKE  HUMORUM. 

From  a  study  made  in  1875. 

A  view  of  one  of  the  lunar  plains  [47c],  or  so-called  seas  (Maria),  with  an  encircling 
mountainous  wall  [45a]  consisting  of  volcano-like  craters  [45c,  46c]  in  various  stages  of  sub- 
sidence and  dislocation  [48a].  The  sun-light  coming  from  the  west  casts  strong  shadows 
from  all  the  elevations  eastward,  and  is  just  rising  on  the  terminator  [44c],  where  the  rugged 
structure  of  the  Moon's  surface  is  best  seen.  The  lighter  portions  are  the  more  elevated 
mountainous  tracts  and  crater  summits  [45a].  The  detailed  description  of  this  Plate 
given  in  the  body  of  the  MANUAL  [51-2]  is  repeated  here  for  convenience  of  reference:  The 
Mare  Humorum,  or  sea  of  moisture,  as  it  is  called,  is  one  of  the  smaller  gray  lunar 
plains.  Its  diameter,  which  is  very  nearly  the  same  in  all  directions,  is  about  270  miles,  the 
total  area  of  this  plain  being  about  50,000  square  miles.  It  is  one  of  the  most  distinct  plains  of 
the  Moon,  and  is  easily  seen  with  the  naked  eye  on  the  left-hand  side  of  the  disk.  The  floor 
of  the  plain  is,  like  that  of  the  other  gray  plains,  traversed  by  several  systems  of  very  ex- 
tended but  low  hills  and  ridges,  while  small  craters  are  disseminated  upon  its  surface. 
The  color  of  this  formation  is  of  a  dusky  greenish  gray  along  the  border,  while  in  the  in- 
terior it  is  of  a  lighter  shade,  and  is  of  brownish  olivaceous  tint.  This  plain,  which  is  sur- 
rounded by  high  clefts  and  rifts,  well  illustrates  the  phenomena  of  dislocation  and  subsidence. 
The  double-ringed  crater  Vitello,  whose  walls  rise  from  4,000  to  5,000  feet  in  height,  is  seen 
in  the  upper  left-hand  corner  of  the  gray  plain.  Close  to  Vitello  at  the  east  is  the  large 
broken  ring-plain  Lee,  and  farther  east,  and  a  little  below,  is  a  similarly  broken  crater 
called  Doppelmayer.  Both  of  these  open  craters  have  mountainous  masses  and  peaks  on 
their  floor,  which  is  on  a  level  with  that  of  the  Mare  Humorum.  A  little  below,  and  to 
the  left  of  these  objects,  is  dimly  seen  a  deeply  imbedded  oval  crater,  whose  walls  barely 
rise  above  the  level  of  the  plain.  On  the  right-hand  side  of  the  great  plain  is  a  long  fault, 
with  a  system  of  iracture  running  along  its  border.  On  this  right-hand  side  may  be  seen 


KEY  TO    THE  PLATES.  163 

a  part  of  the  line  of  the  terminator,  which  separates  the  light  from  the  darkness.  Towards 
the  lower  right-hand  corner  is  the  great  ring-plain  Gassendi,  55  miles  in  diameter,  with  its 
system  of  fractures  and  its  central  mountains,  which  rise  from  3,000  to  4,000  feet  above  its 
floor.  This  crater  slopes  towards  the  plain,  showing  the  subsidence  to  which  it  has  been 
submitted.  While  the  northern  portion  of  the  wall  of  this  crater  rises  to  10,000  feet,  that  on 
the  plain  is  only  500  feet  high,  and  is  even  wholly  demolished  at  one  place  where  the  floor 
of  the  crater  is  in  direct  communication  witlf  the  plain.  In  the  lower  part  of  the  sea,  and  a 
little  to  the  west  of  the  middle  line,  is  found  the  crater  Agatharchides,  which  shows  below  its 
north  wall  the  marks  of  rills  impressed  by  a  flood  of  lava,  which  once  issued  from  the  side  of 
the  crater.  On  the  left-hand  side  of  the  plain  is  seen  the  half-demolished  crater  Hippalus 
resembling  a  large  bay,  which  has  its  interior  strewn  with  peaks  and  mountains.  On  this 
same  side  can  be  seen  one  of  the  most  important  systems  of  clefts  and  fractures  visible  on 
the  Moon,  these  clefts  varying  in  length  from  150  to  200  miles. 
Approximate  scale,  15  miles— 1  inch. 

PLATE  VII.— PARTIAL  ECLIPSE  OF  THE  MOON. 
Observed  October  24,  1874. 

A  view  of  the  Moon  partially  obscured  by  the  Earth's  shadow  [53a],  whose  outline  gives 
ocular  proof  of  the  earth's  rotundity  of  form.  The  shadowed  part  of  the  Moon's  surface  is 
rendered  visible  by  the  diffused  sun-light  refracted  upon  it  from  the  earth's  atmosphere 
[55a].  Its  reddish  brown  color  is  due  to  the  absorption,  by  vapors  present  near  the  earth's 
surface  [55a],  of  a  considerable  part  of  this  dim  light.  On  both  the  obscured  and  illu- 
minated tracts  the  configurations  of  the  Moon's  surface  are  seen  as  with  the  naked  eye 
[566].  The  craters  [45c]  appear  as  distinct  patches  of  lighter  color,  and  the  noticeably 
darker  areas  are  the  depressed  plains  or  Maria  [47c].  The  large  crater  Tycho,  at  the  lower 
part  of  the  disk,  is  the  most  prominent  of  these  objects,  with  its  extensive  system  of  radiating 
streaks  [49a].  The  largest  crater  above  is  Copernicus,  at  the  left  of  which  is  Kepler  and  still 
above  are  Arisiarchus  and  Herodotus  appearing  as  if  blended  in  one.  Above  and  at  the  left 
of  the  great  crater  Tycho,  the  first  dark  tract  is  the  Mare  Humorum  of  Plate  VI.,  seen  in  its 
natural  position  [vi.c],  with  the  crater  Gassendi  at  its  northern  (upper)  extremity  and  Vitetto 
on  its  southern  (lower)  border  [516].  The  advancing  border  of  the  shadow  appears,  as 
always,  noticeably  darker  than  the  remainder,  an  effect  probably  of  contrast.  The  illu- 
minated segment  of  the  Moon's  disk  has  its  usual  appearance,  the  lighter  portions  being  the 
more  elevated  mountainous  surfaces,  and  the  dark  spaces  the  floors  of  extensive  plains. 

Approximate  scale,  140  mues—l  inch. 

PLATE  VIII.— THE    PLANET    MAES. 
Observed  September  3,  1877,  at  llh.  55m.  P.  M. 

A  view  of  the  southern  hemisphere  of  Mars  [64a],  when  in  the  most  favorable  position  for 
observation  [616],  and  when  exceptionally  free  from  the  clouds,  which  very  frequently  hide  its 
surface  configurations  [636].  Since,  of  all  the  planets,  Mars  is  most  like  the  earth  [71c], 
Plate  VIII.  may  give  a  fair  idea  of  the  appearance  of  our  globe  to  a  supposed  observer  on  Mars. 
The  dark  gray  and  black  markings  [63c],  are  regarded  as  tracts  of  water  [656],  or  of  some 
liquid  with  similar  powers  of  absorbing  light;  and,  for  the  same  reason,  the  lighter  portions, 
of  a  prevailing  reddish  tint  [70c],  are  supposed  to  be  bodies  of  land  [656],  while  the  bright 
white  portions  are  variously  due  to  clouds  [686],  to  polar  snow  or  ice  [656,  676],  and  the  bright 
rim  of  white  along  the  limb,  to  the  depth  of  the  atmosphere  through  which  the  limb  is  seen 


164  APPENDIX. 

£70c].  The  chief  permanent  features  of  the  planet's  surface  have  been  named  in  honor  of 
various  astronomers. 

The  large  dark  tract  on  the  left  is  De  La  Rue  Ocean,  the  isolated  oval  spot  near  the  centre  is 
Terby  Sea,  and  on  the  right  is  the  western  end  of  Maraldi  Sea,  with  strongly  indented  border. 
Directly  north  of  (below)  De  La  Eue  Ocean,  is  Maedler  Continent;  above  it  stretches  Jacob  Land; 
and  surrounding  Terby  Sea  is  Secchi  Continent.  Extending  into  the  centre  of  De  La  Hue  Ocean  is 
a  curious  double  peninsula,  called,  in  consequence  of  the  dimness  of  former  observations, 
Hall  Island  [69a].  The  sharply  defined,  white-crested  northern  borders  of  De  La  Rue  Ocean 
and  Maraldi  Sea  may  indicate  the  existence  there  of  lofty  coast  ranges,  more  or  less  con- 
stantly covered  with  opaque  clouds  [636]  strongly  reflecting  light  [656].  The  white  spot  in 
the  centre  of  Maedler  Continent,  of  a  temporary  nature,  has  a  similar  explanation  [69c].  The 
intervals  of  olivaceous  gray  on  Secchi  Continent  and  elsewhere  may  perhaps  be  ascribed  [65c] 
to  the  flooding  and  drying  up  of  marshes  and  lowlands,  as  these  markings  have  been  ob- 
served to  vary  somewhat  in  connection  with  the  change  of  seasons  on  Mars  [71c].  The  green- 
ish tints  observed  along  the  planet's  limb,  alike  on  the  darker  and  lighter  surfaces,  are  prob- 
ably due  to  an  optical  effect,  the  green  being  complementary  to  the  prevailing  red  of  the  disk. 
The  brilliant  oval  white  spot  near  the  southern  (upper)  pole  of  the  planet  is  a  so-called  polar 
spot  [65-67],  in  all  probability  consisting  of  a  material  similar  to  snow  or  ice  [676]  and  here 
observed  in  the  midst  of  a  dark  open  sea  [67c]. 

Approximate  scale,  300  mites— 1  inch. 

PLATE  IX.— THE   PLANET  JUPITEK. 

Observed  November  1,  1880,  at  9ft.  30m.  P.  M. 

This  planet  is  perpetually  wrapped  in  dense  clouds  [756]  which  hide  its  inner  globe  from 
view.  The  drawing  shows  Jupiter's  outer  clouded  surface  with  its  usual  series  of  alternate 
light  and  dark  belts  [746],  the  disk  as  a  whole  appearing  brighter  in  the  centre  than  near  the 
limb.  The  darker  gray  and  black  markings  [74c]  indicate  in  general  the  lower  cloud-levels ; 
that  is,  partial  breaks  or  rifts  in  the  cloudy  envelope,  whose  prevailing  depth  apparently 
exceeds  four  thousand  miles  [79c-80c].  While  the  deepest  depression  in  the  cloudy  envelope 
is  within  the  limits  of  the  Great  Bed  Spot  [77a],  the  vision  may  not  even  here  penetrate 
very  deeply.  Two  of  Jupiter's  four  moons  [77c]  present  bright  disks  [78c]  near  the 
planet's  western  limb,  and  cast  their  shadows  [79a]  far  eastward  on  the  disk,  that  of  the 
"  second  satellite  •'  falling  upon  the  Bed  Spot  [82a].  On  the  Bed  Spot  are  seen  in  addition 
two  small  black  spots  [77a],  no  explanation  of  which  can  yet  be  offered.  The  broad  white 
ring  of  clouds  bordering  the  Bed  Spot  [776]  appeared  in  constant  motion.  The  central,  or 
equatorial  belt  [74c],  shows  brilliant  cloudy  masses  of  both  the  cumulus  and  stratus  types,  and 
the  underlying  gray  and  black  cloudy  surfaces  are  pervaded  with  the  pinkish  color  charac- 
teristic of  this  belt.  The  dark  circular  spots  on  the  wide  white  belt  next  north  showed  in  their 
mode  of  formation  [76c]  striking  resemblances  to  sun-spots  [82a].  They  afterward  coalesced 
into  a  continuous  pink  belt.  The  diffusion  of  pinkish  color  over  the  three  northernmost  dark 
bands,  as  here  observed,  is  unusual  [75a].  About  either  pole  is  seen  the  uniform  gray  seg- 
ment [75a]  or  polar  cap.  The  equatorial  diameter  is  noticeably  longer  [73c]  than  the  polar 
diameter,  a  consequence  of  the  planet's  extraordinary  swiftness  of  rotation.  To  tho  same 
cause  may  also  be  due  chiefly  the  distribution  of  the  cloudy  belts  parallel  to  the  planet's 
equator  [746],  though  the  analogy  of  the  terrestrial  trade-winds  fails  to  explain  all  the  ob- 
served phenomena. 

Approximate  scale,  5,500  miles— 1  inch. 


KEF  TO   THE  PLATES.  165 

PLATE   X.— THE  PLANET  SATUKN. 

Observed  November  30,  1874,  at  5h.  50m.  P.  M. 

Saturn  is  unique  amongst  the  planets  in  that  its  globe  is  encircled  by  a  series  of  concen- 
tric rings  [86c], which  lie  in  the  plane  of  its  equator,  and  consist,  according  to  present  theories, 
of  vast  throngs  of  minute  bodies  revolving  about  the  planet,  like  so  many  satellites,  in  closely 
parallel  orbits  [97c].  The  globe  of  Saturn,  like  that  of  Jupiter,  is  surrounded  by  cloudy  belts 
parallel  to  its  equator  [85c].  The  broad  equatorial  belt,  of  a  delicate  pinkish  tint,  is  both 
brighter  and  more  mottled  [85c]  than  the  narrower  yellowish  white  belts,  which  alternate  with 
darker  belts  of  ashy  gray  [86a]  on  both  the  north  and  south  sides,  but  are  seen  here  only  on 
the  northern  side.  The  disk  has  an  oval  shape,  owing  to  the  extreme  polar  compression  of 
the  globe  [856]. 

The  outer,  middle  and  inner  rings,  with  their  various  subdivisions  [86c],  are  clearly  shown 
in  Plate  X.,  and  are  best  seen  on  the  so-called  ansce,  or  handles,  projecting  on  either  side. 
The  gray  outer  ring  is  separated  by  the  dusky  pencil  line  [86c]  into  two  divisions,  both  of  which 
appear  slightly  mottled  on  the  ansae,  as  if  with  clouds  [87a].  The  middle  ring  has  three  sub- 
divisions which  are  clearly  distinguishable,  although  separated  by  no  dark  interval,  viz.,  a 
brilliant  white  outer  zone,  distinctly  mottled,  as  seen  on  the  extremities  of  the  ansae,  and  two 
interior  zones  of  gradually  diminishing  brightness  [87a].  The  gauze  or  dusky  ring  is  seen  at 
its  full  width  on  the  ansae,  but  on  the  background  of  the  strongly  illuminated  globe  only  its 
outer  and  presumably  denser  border  [87c]  is  visible.  The  sha'dow  of  the  globe  on  the  rings 
[886]  is  seen  on  the  lower  portion  of  the  eastern  ansa.  The  shadow  on  the  dusky  ring 
[88c]  is  with  difficulty  perceptible;  the  shadow  on  the  middle  ring  is  slightly  concave  toward 
the  planet,  which  concavity  is  abruptly  increased  on  the  outer  zone  of  this  ring  [89a]  ;  while 
the  shadow  on  the  outer  ring  slants  away  from  the  globe.  These  appearances  are  fully 
accounted  for  by  supposing  a  general  increase  of  level  from  the  inner  edge  of  the  dusky  ring 
to  the  outer  margin  of  the  middle  ring,  and  a  uniform  lower  level  on  the  outer  ring.  Other 
observers  have  regarded  the  deflection  of  the  shadow  as  an  effect  of  irradiation  [89-90]. 
The  inner  margin  of  the  double  outer  ring  presents  on  both  ansse  a  number  of  slight 
indentations,  which,  if  not  actual  irregularities  in  the  contour  of  this  ring,  may  be  explained 
as  shadows  caused  by  elevations  on  the  outer  border  of  the  middle  ring,  or  possibly  by  over- 
hanging clouds. 

Approximate  scale,  6,500  miles— I  inch. 

PLATE  XI.— THE   GEE  AT  COMET  OF  1881. 
Observed  on  the  night  of  June  25-26,  at  Ih.  30m.  A.  M. 

A  view  of  the  comet  1881,  III.,  drawn  as  if  seen  with  the  naked  eye,  the  minute  details, 
however,  being  reproduced  as  seen  with  the  telescope  [vi.c].  The  star-like  nucleus  [1036]  is 
attended  by  four  conical  wings  [104a]  which  cause  it  to  appear  diamond-shaped.  The  coma 
[1056]  appears  double,  the  brilliant  inner  coma,  immediately  enveloping  the  nucleus,  being 
surrounded  by  a  fainter  exterior  coma  [106a],  which  has  a  noticeable  depression  correspond- 
ing to  that  of  the  inner  edge  of  the  principal  coma.  The  tail  is  divided  lengthwise  by  a  dark 
rift  [1066]  and  is  brightest  on  its  convex  or  forward  side  [106c].  An  inner  portion  of  the  tail, 
brighter  than  the  rest,  is  more  strongly  curved,  as  if  by  solar  repulsion  [105c,  113].  Stars,  are 
seen  through  the  brighter  parts  of  the  tail,  as  they  may  be  seen  even  through  the  coma  and 
nucleus,  with  little  diminution  of  their  light  [1116]. 


1G6  APPENDIX. 

PLATE  XII.-THE   NOVEMBEK  METEORS. 

As  observed  on  the  night  of  November  13-14,  1868. 

A  partially  ideal  view  of  the  November  Meteors  [116c],  combining  forms  observed 
[vii.a]  at  different  times  during  the  night  of  Nov.  13th,  1868.  It  is  not,  however,  a  fanciful 
view  [118c],  since  a  much  larger  number  of  meteors  were  observed  falling  at  once  during  the 
shower  of  November,  1833,  and  at  other  times  [1166].  The  locality  of  the  observation  is  shown 
by  the  Polar  Star  seen  near  the  centre  of  the  Plate,  and  Cassiopeia's  Chair  at  the  left.  The 
general  direction  of  the  paths  of  the  meteors  is  from  the  north-east,  the  radiant  point  [120a] 
of  the  shower  having  been  in  the  constellation  Leo  [121a],  beyond  and  above  Ursa  Major. 
While  the  orbits  of  the  meteors  are,  in  general,  curved  regularly  and  slightly,  [117a],  several 
are  shown  with  very  eccentric  paths  [1176],  among  them  one  which  changed  its  course  at  a  sharp 
angle.  In  the  upper  left-hand  corner  appear  two  vanishing  trails  of  the  "  ring-form,"  and  sev- 
eral others  still  further  transformed  into  faint  luminous  patches  of  cloud  [1186].  Bed,  yellow, 
green,  blue  and  purple  tints  were  observed  in  the  meteors  and  their  trails  [1186],  as  repre- 
sented in  the  Plate. 

PLATE  XIII.—  PART   OF  THE  MILKY  WAY. 

From  a  study  made  during  the  years  1874,  1875  and  1876. 

The  course  of  the  portion  of  the  Galaxy  [130]  represented  in  Plate  XIII.  is  as  follows  : 
From  Cassio.peia's  chair,  three  bright  stars  of  which  appear  at  the  upper  edge  of  the  Plate, 
the  Galaxy,  forming  two  streams,  descends  south,  passing  partly  through  Lacerta  on  the  left, 
and  Cepheus  on  the  right  ;  at  this  last  point  it  approaches  nearest  to  the  Polar  Star,  which  is 
itself  outside  of  the  field  of  view.  Then  it  enters  Cygnus,  where  it  becomes  very  complicated 
and  bright,  and  where  several  large  cloudy  masses  are  seen  terminating  its  left  branch,  which 
passes  to  the  right,  near  the  bright  star  Deneb,  the  leader  of  this  constellation.  Below  Deneb, 
the  Galaxy  is  apparently  disconnected  and  separated  from  the  northern  part  by  a  narrow, 
irregular  dark  gap.  From  this  rupture,  the  Milky-way  divides  into  two  great  streams,  sepa- 
rated by  an  irregular  dark  rift.  An  immense  branch  extends  to  the  right,  which,  after  having 
formed  an  important  luminous  mass  between  the  stars  Gamma  and  Beta,  continues  its  south- 
ward progress  through  parts  of  Lyra,  Vulpecula,  Hercules,  Aquila  and  Ophiuchus,  where  it 
gradually  terminates  a  few  degrees  south  of  the  equator.  The  main  stream  on  the  left,  after 
having  formed  a  bright  mass  around  Epsuon  Oygni,  passes  through  Vulpecula  and  then 
Aquila,  where  it  crosses  the  equinoctial  just  below  the  star  Eta.,  after  having  involved  in  its 
nebulosity  the  bright  star  AUair,  the  leader  of  Aquila.  In  the  southern  hemisphere  the 
Galaxy  becomes  very  complicated  and  forms  a  succession  of  very  bright,  irregular  masses, 
the  upper  one  being  in  Scutum  Sobieskii,  while  the  others  are  respectively  situated  in  Sagit- 
tarius and  in  Scorpio  ;  the  last,  just  a  little  above  our  horizon,  being  always  considerably 
dimmed  by  vapors.  From  Scutum  Sobieskii,  the  Galaxy  expands  considerably  on  the  right, 
and  sends  a  branch  into  Scorpio,  in  which  the  fiery  red  star  Antares  is  somewhat  involved. 
In  the  upper  left-hand  corner  of  the  Plate,  at  some  distance  from  the  Milky-way,  is  seen 
dimly  the  Nebula  in  Andromeda,  which  becomes  so  magnificent  an  object  to  telescopic  view 


PLATE  XIV.  -STAR-CLUSTER  IN   HERCULES. 

From  a  study  made  in  June,  1877. 

In  the  constellation  Hercules  [137c],  a  small  nebulous  mass  is  faintly  visible  to  the  eye 
[1386],  a  telescopic  view  of  which  is  presented  in  Plate  XIV.    It  is  one  of  the  most  beautiful 


KEY  TO   THE  PLATES.  167 

of  the  easily  resolvable  [139a]  globular  clusters  [1396].  The  brilliancy  of  the  centre  gives 
the  cluster  a  distinctly  globular  appearance,  while  the  several  wings  [1406]  curving  in  vari- 
ous directions,  have  suggested  to  some  observers  an  irregularly  spiral  structure  [140c].  The 
large  stars  of  the  cluster  are  arranged  in  several  groups  which  correspond,  in  a  general  way, 
with  the  faintly  luminous  wings. 

PLATE  XV.— THE   GREAT  NEBULA  IN  ORION. 

From  a  study  made  in  the  years  1875-76. 

This  nebula,  which  is  one  of  the  most  brilliant  and  wonderful  of  telescopic  objects, 
readily  visible  to  the  naked  eye  as  a  patch  of  nebulous  light  immediately  surrounding  the 
middle  star  of  the  three  which  form  the  sword  of  Orion,  and  a  little  south  of  the  three  well- 
known  stars  forming  the  belt  [153a].  The  small  stars  in  this,  as  in  other  Plates  of  the  series, 
are  somewhat  exaggerated  in  size,  as  was  unavoidable  with  any  mode  of  reproduction  that 
could  be  employed  [41o].  The  bright  pentagonal  centre  of  the  nebula  [1526]  is  traversed  by 
less  luminous  rifts,  the  several  subdivisions  thus  outlined  being  irregularly  mottled  as  if 
by  bright  fleecy  clouds  [154a].  Toward  the  lower  part  of  this  bright  pentagonal  centre  is  a 
comparatively  dark  space  containing  four  bright  stars  which  form  a  trapezium  and  together 
constitute  the  quadruple  star  Thela  Orionis,  which,  to  the  naked  eye,  appears  as  the  single 
star  in  the  centre  of  the  sword.  On  three  sides  of  the  central  mass  extend  long  bright  wisps, 
whose  curves  fail,  however,  to  reveal  the  spiral  structure  often  attributed  to  this  nebula 
[1546].  On  the  east  a  broad  wing,  with  wave-shaped  inner  border,  stretches  southward 
[1536].  East  of  the  trapezium  are  two  especially  noticeable  dark  spaces.  Close  to  the  main 
nebula  on  the  north-east,  a  small  faint  nebula  surrounds  a  bright  star,  and  a  branch  from 
another  faint  stream  of  nebulous  matter  forming  a  loop  to  the  southward,  encloses  the  nebu- 
lous star  (Iota  Orionis)  shown  at  the  top  of  the  Plate. 


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